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ASTRONOMY  BY  OBSERVATION. 


SPECTRA    OF    VARIOUS    SOURCES    OF  LIGHT. 


HIM  IIIIHJMP' 
I   I  I 


ii  (L       1.1  B 


CsBCsa 


»«x      Bad.  Bag 


111 


rnu  rnn 


KDti  KbO. 


.  The  Saris  edge.  3.  Soduim.4  Potassium.  5.  LtMiun.  6  (hesuan.  7  Rabidwm.  &  Thallium. 
S-CbUdusn.  \&Stmtfuum.\l£anum\2.fndujm  M>.Pko'sphorus.  14 Hydrogen. 


NEW  YORK,  D.  APPLETON  aC9 


ASTRONOMY 


BY     OB  S  E  RVATION 


AN     ELEMENTARY      TEXT-BOOK 
FOR     HIGH-SCHOOLS     AND     ACADEMIES 


BY 


ELIZA    A.   BOWEN 


NEW  YORK 
D.    APPLETON    AND    COMPANY 

I,    3,  AND   5    BOND  STREET 
1888 


TO 

MRS.    ZOE    DANA    UNDERBILL, 

WHOSE    ENLIGHTENED    APPRECIATION    OF    OBSERVATION-STUDY 

STIRRED   ME   UP   TO   WRITE   THIS   BOOK, 

AND   WHOSE   STEADY   FRIENDLY   ENCOURAGEMENT 

SUSTAINED   ME   THROUGH   SEVERAL   YEARS   OF   EXPERIMENTAL  WORK. 


+ 


COPYRIGHT,  1886, 
BY  D.   APPLETON   AND  COMPANY. 


PREFACE. 


THIS  book  has  grown  out  of  actual  school-work,  in  which  it  was  the  teacher's  object  to  make  pupils 
studying  elementary  astronomy  observe  and  think. 
The  following  are  its  chief  peculiarities  : 

1.  An  efficient,  easy,  well-tried  plan  for  teaching  the  constellations  is  described.     Its  use  will  obviate 
the  necessity  of  a  teacher  doing  work  out  of  school-hours,  by  enabling  students  to  become  independent 
observers. 

2.  Careful  directions  are  given  when,  how,  and  where  to  find  the  heavenly  bodies.     Their  motions  are 
described  in  the  order  in  which  they  can  be  seen  by  an  observer,  and  in  familiar  language.     Thus,  the  text- 
book may  be  a  guide  to  the  observation  of  beginners. 

3.  The  student  is  excited  to  thought.     Facts  are  stated  first ;  theory  afterward,  as  a  deduction  from  the 
facts.     The  selection  of  subjects  for  the  student's  thinking  is  a  little  different  from  that  of  other  school 
astronomies.    The  general  principle  governing  this  selection  is  to  make  the  student  understand  what  he  can 
see.     The  author  has  also  desired  in  some  degree  to  separate  the  work  of  the  elementary  teacher  from  that 
of  the  college  professor.    To  observe  the  facts  from  which  Copernicus  argued ;  to  discuss  them  ;  to  perceive 
how  they  prove  his  conclusions ;  to  use  in  this  the  simpler  geometrical  conceptions,  seem  to  the  author 
suitable  elementary  work.     It  would  be  excellent  elementary  work  if  it  could  be  combined  with  some  prac- 
tical angular  measurements  of  a  simple  kind  on  the  heavens. 

Practically,  neither  the  college  professor  nor  the  elementary  teacher  can  draw  a  clear  line  separating 
their  work.  It  would  be  very  unfortunate  if  these  few  words  suggest,  for  judging  of  the  author's  humble 
book,  a  severe  abstract  rule,  which  the  practical  condition  of  the  schools  makes  it  impossible  for  any  ele- 
mentary teacher  to  observe. 

The  author  knows  high-school  work  well.  The  thinking  of  this  book  is  fairly  within  the  comprehension 
of  the  students  of  our  reasonably  good  schools.  The  professed  study  of  the  theory  of  astronomy,  in  which 
there  is  no  thinking,  is  a  counterfeit. 

The  book  presumes  in  those  who  study  it  a  little  knowledge  of  elementary  geometry,  and  of  the  refrac- 
tion, reflection,  and  dispersion  of  light.  As  all  high-schools  teach  as  much  of  these  subjects  as  is  needed,  it 
seemed  better  to  take  thus  much  knowledge  for  granted,  than  to  make  a  digression  for  the  purpose  of  pre- 


senting it. 


The  author  has  probably  experimented  as  much  as  anybody  in  teaching  elementary  pupils  by  observa- 
tion and  by  questions  which  drew  them  out  to  discuss  theory.  But  much  review  is  always  required,  and, 
in  order  to  save  time,  a  text-book  is  necessary  for  it.  Also,  we  can  not  ring  up  the  planets,  like  scenes  in 
a  theatre,  to  be  studied  during  the  school-session,  and  therefore  the  theory  must  sometimes  be  taught  in 
advance  of  actual  observation.  The  author  hopes  the  book  will  be  useful  for  these  purposes  to  teachers  who 
use  improved  methods. 

In  writing  it,  another  class  of  teachers  has  been  much  in  view.  In  our  high-schools  and  academies  the 
teaching  of  astronomy  is  often  in  the  hands  of  instructors  who  have  little  or  no  practical  knowledge  of  the 
science.  They  are  usually  intelligent  persons  who  would  improve  if  the  text-book  were  a  guide  to  obser- 
vation. 

Another  class  of  persons  has  also  been  in  view.  Of  late  years  a  great  spirit  of  study  has  arisen  among 
young  people  out  of  school.  Their  situation  is  very  favorable  for  the  study  of  astronomy,  since  they  are 
not  bound  by  school  hours  and  sessions,  but  can  take  the  planets  when  they  come.  It  is  hoped  that  they 
will  be  encouraged  to  study  the  heavens  if  a  sufficient  guide  to  observation  is  furnished.  No  study  lends 
itself  better  to  teaching  by  correspondence  than  astronomy. 

BEECHCROFT  SCHOOL,  TENN.,  Dec.  20,  1885. 


CONTENTS. 


INTRODUCTION. 

PACE 

An  Easy  and  Effective  Way  to  learn  and  teach  the  Constella- 
tions— How  to  direct  the  Observations  of  Pupils  .  .  5 

PART    I. 

CHAPTER    I. 
Facts  necessary  to  know  before  beginning  Observation      .         .        9 

CHAPTER   II. 

The  Diurnal  Motion  of  the  Stars — How  to  observe  it — How  to 
interpret  it— The  Celestial  Sphere— The  Equinoctial  and 
Horizon  Systems  of  Circles 10 

CHAPTER    III. 

The  Annual  Motion  of  the  Stars,  and  how  to  observe  it — The 

Annual   Motion   of  the   Sun,  and   how  to   observe  it — The 
Connection  and  Interpretation  of  these  Motions  .        ,         .       14 

CHAPTER   IV. 

The  Inequality  of  Days  and  Nights  observed  and  interpreted — 
The  Change  of  Seasons  observed  and  interpreted — Unequal 
Motions  of  Sun  and  Earth — Precession  of  the  Equinoxes — 
Time 23 

CHAPTER   V. 
The  Moon  and   her  Motions— How  to  observe  them — How  to 

interpret  them — Eclipses .31 

CHAPTER   VI. 
The   Planets   and   their   Motions — How  to   observe  them — How 

to  interpret  them 39 

CHAPTER   VII. 

The  Attraction  of  Gravitation— The  Mathematical  Measurement 
of  a  Physical  Force— Copernicus  and  the  Solar  Theory— 


Tycho    Brahe's    Work— Kepler's    Laws — Newton    and    the 
Solar  Theory — Celestial  Measurements  described          .        .      48 

PART    II. 

CHAPTER   VIII. 

The  Sun — His  Apparent  and  Real  Magnitudes — His  Volume, 
Density,  Light,  Heat,  etc. — The  Sun,  the  Spectroscope,  and 
the  Telescope — The  Photosphere — The  Chromosphere — The 
Corona 52 

CHAPTER    IX. 

The  Planets — Their  Figures,  Volumes,  Densities,  Orbits,  Revo- 
lutions, Distances,  Moons,  etc.— The  Planets  and  the  Moon, 
with  the  Spectroscope  and  Telescope — Their  Physical  Con- 
stitution   62 

CHAPTER    X. 
Meteoroids  and  Comets — The  Zodiacal  Light      .        .        .        .71 

PART    III. 

CHAPTER   XI. 
The  Heavens  beyond  the  Solar  System 75 

APPENDICES. 

APPENDIX  A. 

Account  of  the  Constellations,  alphabetically  arranged  for  Refer- 
ence   80 

APPENDIX   B. 
The  Telescope 86 

APPENDIX   C. 
Observation  of  Meteors  and  Comets 87 


INDEX 


88 


INTRODUCTION. 

For  Teachers  and  Private  Learners. 


HOW    TO    LEARN    AND    TEACH    THE    CONSTELLATIONS.' 


WE  can  not  help  seeing  the  motions  of  the  heavenly  bodies, 
unless,  like  Bunyan's  man  with  the  muck-rake,  we  steadily  look 
down.  A  professed  study  of  astronomy  which  does  not  make 
us  recognize  these  bodies  and  their  motions,  when  we  see  them, 
may  not  be  useless,  but  it  bears  some  marks  of  counterfeit 
knowledge.  A  mere  acquaintance  with  the  constellations  is  a 
very  subordinate  part  of  the  knowledge  here  recommended,  but 
it  has,  besides  its  own  value,  a  good  deal  of  importance  as  a 
prerequisite  to  the  rest. 

There  are  two  disadvantages  in  the  way  of  a  teacher  who 
wishes  to  make  his  pupils  in  astronomy  really  study  nature.  He 
can  not  be  with  them  at  night  without  inconvenience,  and  work 
outside  of  school-hours.  This  is  the  first  difficulty.  Another 
is,  the  impossibility  of  having  the  planets  brought  out  for  inspec- 
tion when  we  want  them,  and  also  because,  when  they  come, 
their  motions  are  very  slow. 

After  struggling  a  good  deal  with  these  disadvantages  just 
mentioned,  I  found  that  by  making  young  students  learn  the 
heavens  as  they  do  the  maps  of  the  United  States  or  Europe, 
viz.,  by  map-drawing,  the  whole  of  the  first  difficulty  was  ob- 
viated, and  a  large  part  of  the  second.  They  found  the  star- 
groups  without  trouble,  for  themselves.  They  were  trained  in 
observation,  and  they  learned  a  great  many  of  the  motions  inci- 
dentally. They  became  independent  observers,  and  were  so 
much  interested  in  the  practical  study,  that  there  was  no  doubt 
they  would  see  and  study  for  themselves  whatever  they  could 
not  see  during  the  school-session. 

No  doubt  a  great  many  teachers  can  carry  on  this  work 
quite  as  well  as  I  can,  or  better  ;  but,  for  the  benefit  of  those  who 
can  not,  I  will  describe  it  somewhat  in  detail. 

The  brighter  stars  in  all  the  constellations,  when  joined  by 
imaginary  lines,  form  figures  which  are  very  easily  recognized 
when  once  suggested.  They  are  thus  drawn  in  dotted  lines  on 
the  maps  attached  to  this  book.  If  the  student  takes  one  figure 
at  a  time,  and  draws  it  repeatedly,  until  he  gains  the  power  to 
reproduce  it  on  paper  from  memory,  rapidly,  without  erasures, 
and  with  a  fair  degree  of  rough  correctness,  he  can  easily  find 
it  for  himself  if  it  is  a  conspicuous  constellation,  and  he  has 
some  general  idea  in  what  part  of  the  sky  to  look  for  it.  And 
even  if  it  is  not  a  conspicuous  constellation,  he  can  easily  find 
it,  if  it  is  adjacent  to  a  constellation  already  known,  and  in  a 


known  direction  from  it.  Thus,  beginning  with  conspicuous 
constellations,  and  proceeding  to  adjacent  ones,  and  then  to 
others  adjacent  to  the  last,  they  are  all  easily  learned. 

But  this  plan  of  teaching  by  map-drawing  is  not  a  mere 
substitute  for  what  is  called  "  pointing  out  the  constellations," 
and  is  not  adopted  and  recommended  only  because  the  other 
course  is  inconvenient  or  impossible  when  teacher  and  class 
lodge  in  different  houses.  It  is  so  much  the  easiest  way,  that 
when  instractor  and  pupils  are  already  together  in  the  evening, 
and  the  latter  wish  to  learn  the  star-groups,  the  teacher  will  do 
well  to  sit  in  the  house  (as  I  have  often  done),  draw  the  figures 
on  paper  or  a  blackboard,  and  send  students  out  to  look  for 
them.  At  first  they  feel  helpless,  and  object ;  but  a  little  trial 
soon  convinces  them  how  easy  it  is.  If  any  find  a  difficulty, 
the  others  help  them  ;  and  thus  a  teacher  is  spared  additional 
work. 

But  mere  drawing  by  the  teacher  is  not  sufficient,  nor  is  that 
the  plan  recommended  here.  By  the  method  described  in  the 
two  preceding  paragraphs,  it  is  easy  to  have  star-groups  recog- 
nized at  the  time,  but  they  will  not  be  remembered,  and  must 
be  taught  again.  Drawing  by  the  pupil  not  only  makes  it  easier 
to  find  them ;  they  can  hardly  be  fixed  permanently  in  knowl- 
edge during  the  school-study,  except  by  repeated  drawing  from 
memory.  Whenever  students  have  a  few  leisure  moments,  they 
should  be  called  upon  to  cover  the  blackboard  with  these  fig- 
ures, or  to  reproduce  them  rapidly  upon  paper. 

In  order  to  secure  repeated  observation,  it  is  best  to  have 
the  groups  learned  one  at  a  time  ;  for,  whenever  the  student  goes 
out  to  find  a  new  group,  he  is  sure  to  look  again  at  the  old  ones. 
The  groups  are  both  fixed  in  memory,  and  the  habit  of  observa- 
tion is  strengthened.  But  the  study  of  the  motions  gives  far 
more  interesting  work  than  the  mere  identification  of  star-groups. 

Any  method  of  map-drawing  already  used  in  a  school  will 
answer  for  this  purpose  when  carried  out  under  the  restrictions 
to  be  hereafter  mentioned.  The  plan  which  I  practice  and  rec- 
ommend is  called  "  drawing  by  dictation,"  now  much  practiced 
in  the  best  schools.  The  teacher  leads,  standing  at  the  black- 
board, while  pupils  draw  at  desks  in  unruled  exercise-books, 
which  should  not  be  used  for  any  other  purpose.  When  a 
teacher  is  wholly  incapable  of  drawing  even  these  simple  fig- 
ures, he  can  select  some  pupil  to  take  the  lead,  who  learns  be- 


ASTRONOMY  BY  OBSERVATION. 


forehand  how  to  draw  this  group.  Sufficient  command  over  a 
pencil  for  this  purpose  is  very  common  in  all  schools.  Of  course, 
the  teacher  is  present  to  direct.  The  leader  at  the  blackboard 
draws  one  of  the  lines  of  the  figure  and  pauses  a  moment,  while 
pupils  follow  with  pencil  and  paper.  Then  he  draws  another 
line,  and  they  follow,  and  so  on  until  all  the  lines  are  finished. 
These  lines  are  dotted  on  the  map,  but,  in  the  exercise  described, 
they  are  drawn  continuous  but  very  lightly.  As  the  leader 
draws,  he  calls  attention  to  the  dimensions  and  positions  of  lines. 
No  erasure  whatever  should  be  allowed.  This  is  an  important 
direction,  for,  unless  it  is  observed,  the  teacher  will  be  discour- 
aged by  the  time  consumed.  It  is  not  necessary  to  have  more 
than  a  rough  correctness,  though,  of  course,  pupils  must  not  be 
encouraged  to  draw  carelessly.  After  the  lines  are  drawn,  they 
must  make  the  stars,  the  leader  calling  attention  to  the  number 
and  position.  In  the  drawings  made  by  leader  and  pupils,  stars 
of  the  first  magnitude  are  best  represented  by  the  symbol  O  ; 
of  the  second,  by  % ;  of  the  third,  by  X ;  of  the  fourth,  by  •. 
In  the  "  Description  of  Constellations,"  in  the  Appendix,  there 
is  given  the  number  of  first  and  second  magnitude  stars  for  each 
figure,  and  other  information  of  value  in  studying  them.  After 
each  constellation  is  drawn,  the  name  should  be  written  two  or 
three  times,  and  also  the  names  of  any  first  and  second  magni- 
tude stars  mentioned  in  the  "  Description."  Then  students 
should  be  called  upon  to  repeat  names  aloud,  separately  and 
in  unison.  Without  this  exercise,  they  struggle  a  good  deal  with 
the  names. 

The  drawing  of  a  constellation  must  be  made  two  or  three 
times  rapidly,  and  then  the  students  are  directed  to  turn  over 
a  leaf  and  draw  from  memory.  The  whole  exercise,  properly 
managed,  need  not  take  more  than  ten  minutes.  It  should  take 
place  on  the  morning  of  a  day  which  looks  as  if  it  would  be  fol- 
lowed by  a  clear  night.  If  rain  and  interruption  occur,  the  ex- 
ercise should  be  repeated  when  again  there  seems  a  chance  for 
observation  at  night.  Each  repetition  takes  less  time.  After 
drawing  a  figure,  the  student  is  always  told  to  turn  it  in  every 
direction,  and  is  warned  that  he  can  not  be  sure  beforehand 
exactly  how  it  will  be  turned. 

There  is  one  important  warning  in  regard  to  students  who 
have  identified  one  constellation  and  are  beginning  to  learn  an 
adjacent  one.  If  a  teacher,  trying  this  method,  fails  in  making 
his  students  learn  the  constellations,  it  will  probably  be  from 
carelessness  about  this  matter  now  to  be  explained.  A  picture 
of  a  constellation  looked  down  upon,  so  reverses  the  directions 
of  the  same  figure  looked  up  to  on  the  heavens,  that  the  student 
who  remembers  the  picture  finds  himself,  as  he  says,  "  turned 
round  "  when  he  looks  up.  If  he  is  merely  studying  a  single 
figure,  occupying  a  very  small  portion  of  the  heavens,  he  can 
in  a  few  moments  recover  his  bearings,  and  recognize  the  group 
sought. 

But,  when  the  study  by  picture  comes  to  be  applied  to  a 
larger  portion  of  the  heavens,  or  more  than  a  single  figure,  unless 
the  student  has  the  additional  aid  now  to  be  explained,  he  will 
get  not  only  "  turned  round,"  but  seriously  confused  and  perhaps 
discouraged  as  to  his  power  to  find  anything  at  all  without  some 


one  at  his  elbow.  After  learning  to  draw  a  group  from  memory, 
the  student  must  make  sure  of  the  direction  in  which  it  lies  from 
the  known  group.  It  should  be  the  teacher's  business  to  drill 
him  on  the  direction  of  the  new  group  from  the  known  one. 
He  should  point  out  the  side  of  the  old  one  on  which  it  is  to 
be  found,  and  he  may  draw  the  two  together. 

After  he  has  made  the  drawing,  and  knows  the  direction  of 
the  new  group  from  the  old  one,  he  should  proceed  rigor- 
ously in  the  following  order  when  he  studies  the  heavens:  i. 
He  finds  the  known  constellation.  2.  Dismissing  from  his 
imagination  all  remembrance  of  figures,  he  finds  carefully 
the  proper  direction  from  the  known  group.  When  he  is 
sure  of  the  direction,  and  not  until  then,  he  recalls  the  new 
figure  which  he  has  drawn,  and  he  usually  finds  it  in  a  few 
minutes. 

The  student  should  be  well  drilled  in  this  order  of  proceed- 
ing, made  to  repeat  and  understand  it. 

The  student  should  not  begin  by  studying  the  maps  of  the 
book.  He  will  think  they  are  intended  to  aid  him  in  finding 
the  direction  of  groups  from  himself  and  the  horizon.  They 
do,  it  is  true,  show  him  the  general  aspect  of  the  heavens  at 
the  times  indicated  on  them.  This  is  the  aspect  of  the  circles 
at  the  time  of  the  equinoxes  and  solstices  ;  and  one  object  of 
the  maps  is,  that  the  student  may,  after  learning  star-groups 
separately,  know  these.  It  is  also  the  object  of  the  maps  to 
show  the  figures  of  the  constellations  and  their  directions  from 
each  other,  and  also  the  position  of  the  various  groups  in  regard 
to  the  ecliptic,  the  equinoctial,  and  colures,  but  not  in  regard 
to  the  observer  and  horizon. 

When  the  first  constellation  is  learned,  the  teacher  (who 
looks  it  out  beforehand)  tells  pupils  where  to  look  for  it ;  and, 
whenever  a  constellation  is  studied,  not  adjacent  to  known  ones, 
this  must  be  repeated.  But,  after  they  learn  a  few,  students 
cease  to  think  of  the  position  in  regard  to  the  inclosing  horizon, 
and  see  that  the  maps  can  be  used  to  learn  new  constellations 
adjacent  to  old  ones.  They  then  disregard  the  relation  to  the 
horizon,  and  often  extend  their  knowledge  in  advance  of  the 
drawing.  But  the  teacher  should  not  stop  the  drawing.  The 
securities  for  permanent  and  accurate  knowledge  are  very  insuf- 
ficient without  it. 

The  direction  of  star-groups  from  the  observer  and  his  hori- 
zon could  not  be  permanently  given  by  maps,  for  these  direc- 
tions are  all  the  time  changing.  It  is  true,  a  globe  can  give 
them  so  that  the  change  will  not  be  noticed  for  a  little  while. 
But  the  globe  can  not  be  made  a  satisfactory  help  in  studying 
the  star-groups  unless  we  use  the  directions  from  adjacent 
groups  as  well  as  directions  from  the  horizon. 

The  dictation  exercise  in  drawing,  which  has  been  recom- 
mended and  described,  is  a  species  of  free-hand  drawing,  which 
affords  valuable  training  to  hand  and  eye.  Students  can  cer- 
tainly learn  the  figures  by  drawing  directly  from  the  maps,  but 
they  will  make  little  cramped  figures,  and  find  it  difficult  to 
draw  any  others,  even  on  the  blackboard. 

I  will  state  that,  in  using  this  book,  I  should  have  pupils 
study  Chapter  I,  and  then  devote  two  weeks  to  study  of  con- 


HOW   TO  LEARN  AND    TEACH   THE  CONSTELLATIONS. 


stellations  by  map-drawing,  before  beginning  Chapter  II.*  It 
is  not  easy  to  have  good  book-study  done  concurrently  with 
drawing  and  identifying  star-groups.  The  knowledge  of  the 
constellations  is  of  indispensable  importance  in  studying  the 
motions  of  the  solar  system  in  nature,  though  it  may  easily  be 
overestimated  as  an  end  in  itself.  When  they  are  known,  the 
student  observes  for  himself  the  motions  described  in  the  text. 
Therefore  it  is  best  to  have  those  groups  that  are  visible  learned 
in  the  beginning.  After  this  it  is  not  necessary  to  take  more 
than  a  few  moments  from  book-study,  in  order  to  call  the  stu- 
dent's attention  to  any  phenomena  in  the  heavens  which  it  is 
desirable  he  should  see. 

Of  course,  as  other  constellations  come  into  the  evening  sky 
in  the  east,  they  must  be  learned.  The  occasional  suspension 
of  book-study  for  one  clear  night  will  be  sufficient  in  order  to 
keep  up  with  changes. 

Where  there  is  no  time  to  have  all  the  constellations  studied 
in  class,  the  polar  and  zodiacal  star-groups,  and  those  contain- 
ing first  and  second  magnitude  stars,  should  be  selected.  As 
the  constellations  of  the  zodiac  are  not  all  conspicuous,  their 
superior  importance  may  be  overlooked  by  an  inexperienced 
teacher.f  They  should  be  learned  as  soon  as  possible.];  When 
the  student  knows  them  separately,  he  should  at  once  begin  to 
draw  them  on  the  ecliptic.  He  should  draw  the  line  first,  and 
then  the  star-groups  on  it  in  correct  order  and  position.  Stu- 
dents should  often  be  required  to  draw  the  zodiac  and  ecliptic 
on  strips  of  paper  in  a  few  moments.  It  is  a  good  plan  to  have 
the  line  occasionally  drawn  on  a  long  blackboard,  and  divided 
into  twelve  parts,  and  to  send  twelve  students  together  to  draw 
figures  rapidly,,  giving  them  no  warning  as  to  which  they  are  to 
draw,  and  timing  them.  But  it  is  not  desirable  to  draw  the  con- 
stellations on  the  equinoctial  and  colures,  until  the  student  has 
been  very  familiar  with  the  ecliptic  for  a  long  time.  The  ecliptic 
is  the  important  circle,  and  an  imperfect  knowledge  of  the  equi- 
noctial and  colures  would  poorly  compensate  for  a  failure  to 
be  very  familiar  with  the  ecliptic.  Therefore,  the  other  circles 
should  not  be  studied  until  there  is  no  possible  chance  of  get- 
ting confused  with  the  ecliptic. 

I  hope  I  may  be  pardoned  for  telling  of  a  test  to  which  I  put 
this  method  of  teaching  constellations.  I  took  a  class  of  fifteen 
children,  whose  average  age  was  twelve  years,  but  of  whom  three 
were  only  eleven  years  old,  and  I  began  to  teach  them  the  con- 
stellations at  the  beginning  of  the  school  year  in  September. 
Until  a  test  was  made  at  the  beginning  of  November,  I  was 
never  with  them  at  night  on  a  single  occasion,  nor  had  they  any 
one  at  home  who  gave  them  help.  They  simply  drew  the  fig- 

*  This  plan  would  be  modified  by  bad  weather,  making  observation  im- 
possible. 

f  The  minuteness  of  these  directions  is  intended  for  this  class,  whom  the 
author  hopes  to  aid.  It  is  no  reflection  upon  others. 

I  i  The  facts  which  show  that  the  earth  and  planets  revolve  round  the  sun  I 
can  be  seen  by  anybody  with  eyes.  The  reasoning  which  establishes  this 
theory,  and  the  connection  of  these  facts  with  it,  are  so  simple  and  plain  that 
any  high-school  student  learning  geometry  can  understand  them  clearly.  Thus 
he  can  look  at  the  changes  in  the  heavens  with  intelligence.  But  this  whole 
result  depends  on  knowing  the  constellations  of  the  zodiac. 


ures  and  stars,  and  were  drilled  as  here  described.  There  was 
a  good  deal  of  bad  weather,  and  another  study  took  the  place 
of  star-learning,  at  least  a  third  of  the  time.  The  lesson  was 
half  an  hour  long.  At  the  first  of  November  they  knew  per- 
fectly every  constellation  which  had  been  visible.  They  could 
tell  where  all  the  first  and  second  magnitude  stars  were  found. 
They  knew  nearly  every  third-magnitude  star  and  a  good  many 
fourth.  They  could  trace  the  ecliptic  in  the  heavens,  or  draw 
it.  They  knew  Algol,  Mira,  Epsilon  Lyrse,  the  star  in  Draco 
which  was  the  pole-star  ;  Var  in  Aquila,  midway  between  the 
poles ;  and  what  made  them  all  remarkable.  They  knew  the 
points  where  the  sun  is  found,  December  2 ad  and  March  2ist, 
and  their  names.  They  found  no  difficulty  whatever  in  learn- 
ing Pisces  and  Aquarius.  Some  had  a  little  trouble  with  Her- 
cules and  Ophiuchus,  but  finally  found  them.  They  noticed 
the  daily  and  annual  motion  of  the  stars,  the  revolution  of  the 
moon  and  her  motion  among  the  stars,  and  they  watched  with 
great  interest  the  motion  of  Venus  among  the  stars.  They  also 
noticed  that  the  constellations  of  the  zodiac  seemed  to  approach 
the  sun  in  succession,  and  other  points  not  necessary  to  men- 
tion. Their  enthusiasm  was  very  great,  and  I  am  sure  they  are 
observers  for  life. 

This  book  is,  of  course,  not  intended  for  children,  and  the 
subject  is  only  mentioned  to  show  what  can  be  done  by  the 
method  here  recommended  of  promoting  observation. 

In  schools,  where  advancement  depends  on  examination, 
observation-work  will  almost  inevitably  suffer,  unless  there  is 
also  an  examination  in  it. 

For  the  benefit  of  inexperienced  teachers,  it  is  well  to  give 
some  advice  in  regard  to  the  constellations  which  it  is  best  to 
begin  with.  Of  course,  the  Great  Dipper,  the  Little  Dipper, 
and  Cassiopeia  should  come  first.  Chapter  II  contains  direc- 
tions how  to  learn  them.  After  these  are  learned,  some  con- 
spicuous constellation  must  be  selected  and  found,  and  the  rest 
of  the  study  proceeds  from  that  by  adjacent  constellations.  For 
the  three  autumn  months,  Pegasus  is  best ;  for  the  winter  months, 
Orion  ;  for  spring,  Leo  Major  ;  for  summer,  Scorpio.  By  find- 
ing these  in  the  "  Description  of  Constellations  alphabetically 
arranged,"  Appendix  A,  the  learner  and  inexperienced  teacher 
will  find  hints  how  to  proceed. 

Most  teachers  without  experience  in  observation-teaching 
will,  when  using  this  book  for  the  first  time,  secure  only  part  of 
the  observation-study.  The  book  can  be  used  for  a  mere  cram- 
book,  and  I  have  endeavored  to  make  it  stimulate  the  student 
even  in  that  case,  and  give  him  power  and  zeal  to  help  himself. 
Every  teacher  should  try  to  secure  some  beginning  of  observa- 
tion, and  with  each  successive  class  the  work  will  increase. 

A  school  year  devoted  to  astronomy  will  give  very  admi- 
rable results  ;  but  with  five  months,  work  can  be  done  well 
worth  undertaking.  It  will  be  good  work  if  a  class  sees  enough 
of  the  daily  motion  of  stars  to  perceive  that  it  is  a  revolution 
with  the  pole  for  a  center ;  enough  of  the  annual  motion  to  see 
that  stars  watched  at  intervals  of  two  or  three  weeks  at  the 
same  hour  move  west,  disappear  beyond  the  western  horizon, 
and  reappear  above  the  eastern  ;  some  motion  of  the  sun  on 


8 


ASTRONOMY  BY  OBSERVATION. 


the  horizon  ;  something  of  the  increasing  or  decreasing  obliquity 
of  the  sun's  rays.  The  moon  comes  so  conveniently  for  ob- 
servation that,  where  it  is  possible,  I  recommend  that  she  be 
studied  through  an  entire  lunar  period  from  night  to  night. 
This  work  trains  in  observation,  and  the  knowledge  in  nature, 
of  the  important  positions,  opposition  and  conjunction,  is  use- 
ful when  the  pupil  studies  the  planets. 

This  observation-study  can  be  directed  without  going  out 
with  students  at  night  at  all. 

No  elementary  science  is  so  independent  of  expensive  ap- 
paratus in  schools  as  astronomy.  The  Copernican  theory  is 
the  groundwork  of  all  formal  treatises  on  astronomy,  and  its 
author  died  before  the  invention  of  the  telescope.  No  tele- 
scope, used  in  ordinary  schools  could  aid  in  its  rational  accept- 
ance, except  by  showing  the  phases  of  Venus  and  the  moons  of 
Jupiter.  The  bearing  of  these  on  the  theory  could  not  be  at 
all  appreciated  by  a  person  who  had  not  previously  studied 
what  can  be  seen  with  the  naked  eye.  It  is,  of  course,  a  pleas- 
ure and  advantage,  after  learning  what  can  be  seen  without  a 
telescope,  to  see  these  objects,  and  also  the  rings  of  Saturn,  the 
surface  of  the  moon,  and  the  resolution  of  some  nebulae  ;  but  I 
doubt  whether  it  is  any  advantage  at  all  until  the  student  has 
used  his  unaided  eyes.  The  most  important  need  for  high- 
schools  seems  to  me  to  be  some  simple  means  of  taking  angular 
measurements  on  the  heavens.  This  brings  out  a  little  the  re- 
lation of  the  study  to  mathematics. 

Of  course,  a  large  part  of  the  information  in  any  elementary 
astronomy  comes  from  investigators  equipped  with  the  refined 
and  complicated  instruments  of  modern  observation.  In  this 
case  ordinary  people  can  not  see  for  themselves,  but  must  take 
the  facts  on  testimony.  But  here  the  phenomena  seen  can  be 
so  well  represented  by  picture  that  ordinary  people  are  under 
no  great  disadvantage  in  following  with  intelligence  the  conclu- 


sions of  astronomers.  Of  course,  we  could  neither  intelligently 
accept  the  evidence  nor  appreciate  the  conclusions  if  we  are 
entirely  ignorant  of  the  principles  on  which  the  instruments  are 
constructed  ;  but  these  can  be  easily  understood  by  the  aid  of 
a  few  inexpensive  mirrors,  lenses,  and  prisms. 

It  is  my  own  experience  that  young  persons  have  a  sort  of 
superstition  about  apparatus  which  looks  complicated,  and 
especially  as  if  it  cost  a  good  deal  of  money.  The  feeling  is 
diametrically  opposed  to  the  spirit  of  investigation,  and  there- 
fore until  they  have  become  possessed  of  that  spirit  it  is  best  to 
teach  them  with  the  simplest  appliances. 

Of  illustrative  apparatus,  a  globe,  with  the  ecliptic  traced  on 
it  in  connection  with  the  equinoctial  system  of  circles,  is  im- 
portant ;  but  there  is  a  very  small,  cheap  terrestrial  globe  now 
much  used  in  primary  schools  which  has  the  ecliptic  traced  on 
it  for  the  purpose  of  using  the  globe  to  show  the  circles  on  the 
heavens.  It  can  be  made  to  answer  the  purpose,  using  an  India- 
rubber  band  for  the  horizon  when  we  wish  to  show  the  angles 
it  makes  with  the  ecliptic.  The  celestial  horizon  in  nature  is 
such  a  well-defined  circle,  and  the  sphere  so  plainly  revolves 
through  it,  that  there  is  not  much  lost  by  the  globe  not  revolv- 
ing through  the  India-rubber  band.  Of  course,  a  good  celestial 
globe  is  a  great  advantage,  provided  it  does  not  take  the  place 
of  nature-study.  Where  we  can  see  for  ourselves,  illustrative 
apparatus  is  useful  only  to  give  definiteness  to  our  ideas  de- 
rived from  observation ;  but  the  study  of  nature  should  come 
first.  I  have  many  times  seen  expensive  illustrative  apparatus 
used  so  as  to  discourage  observation.  This  was  not  intended, 
but  practically  this  was  the  result. 


N.  B. — The  various  figures  on  the  maps  sometimes  occur  on  more  than 
one  map.  For  the  student  to  copy,  it  is  best  to  take  them  from  maps  where 
they  are  found  on  or  near  the  middle  of  the  map. 


NOTE. — In  using  Charts  I,  II,  III,  IV,  it  is  best  to  forewarn  students  that, 
where  figures  must  be  represented  very  small,  as  on  these  charts,  the  stars 
appear  relatively  somewhat  larger  than  in  nature.  A  teacher  will  always  find 
it  wiser  for  students  to  draw  on  a  larger  scale.  The  figures  on  the  chart 
facing  page  10  are  on  a  very  good  scale  for  their  drawings. 

The  fuller  charts  at  the  end  of  the  book  are  for  reference  after  the  stu- 


dent has  learned  the  brighter  stars  in  the  constellations,  by  means  of  Charts 
I,  II,  III,  IV.  The  dotted  lines  show  the  boundaries  of  constellations,  the 
Greek  names  of  stars  are  given,  and  there  is  a  fuller  representation  of  the 
stars.  The  student's  attention  should  be  called  to  the  fact  that  these  charts 
represent  the  northern  and  southern  hemispheres,  the  north  pole  being  at  the 
zenith. 


MAP    I. 

For  Study  of  the  Stars  from  January  aoth  to  April  aoth. 


NORTH 


SCALE   OF    MAGNITUDES 

*  *    •    •    • 

12         3         4         5 


SOUTH 

THE    HEAVENS 

AS  SEEN 

December  22d  at  midnight,  February  4th  at  nine  o'clock, 

January  2Oth  at  ten  o'clock,  February  igth  at  eight  o'clock 


CAUTION.— Be  sure  to  read  the 
directions  for  using  these  maps, 
given  in  the  Introduction,  page  5. 


MAP    II. 

For  Study  of  the  Stars  from  July  22d  to  October  22d. 


NORTH 


SCALE    OF    MAGNITUDES 

*  *    •    •    • 

1        *          3         4         5 


SOUTH 


CAUTION.— Be  sure  to  read  the 
directions  for  using  these  maps, 
given  in  the  Introduction  page  5. 


THE 

June  22d  at  midnight, 
July  22d  at  ten  o'clock, 


HEAVENS 

AS  SEEN 

August  yth  at  nine  o'clock, 
August  23d  at  eight  o'clock. 


ASTRONOMY    BY    OBSERVATION. 


CHAPTER    I. 

PREPARATION    FOR    OBSERVING    THE    FIXED    STARS. 

NOTE. — This  brief  account  is  designed  to  furnish  only  so  much  general 
knowledge  as  is  necessary  to  intelligent  observation-study. 

1.  Fixed  Stars  and  Planets. — Nearly  all  the  stars 
which  can  be  seen  on  a  clear  night  belong  to  the  num- 
ber called  fixed  stars.    These  have  been  classed  in  groups 
called  constellations.     The  fixed  stars  are  so  called  be- 
cause they  do  not  appear  to  change  places  in  regard  to 
each  other.*     They  move  across  the  sky  as  if  they  were 
painted   on  a  revolving  canvas.      There  are  five  stars 
visible  to  the  naked  eye,  and  a  good  many  more  which 
can  be  seen  with  telescopes,  that  change  places  in  regard 
to  each  other  and  the  fixed  stars.     They  are  called  plan- 
ets.    The  five  are  named  Saturn,  Jupiter,  Mars,  Venus, 
and  Mercury. 

2.  Constellations. — The  fixed  stars  were  grouped  into 
constellations   by  ancient  astronomers,  who  also  gave 
the  names  to  nearly  all  the  groups.     The  division  was 
arbitrary,  but  it  is  inconvenient  to  change  it.     The  names 
were  those  of  mythological  persons  and  animals,  and 
there  was  usually  a  story  attached  to  each.     These  tales 
have  nothing  to  do  with  modern  astronomy,  so  they  are 
omitted  here.     They  can  be  learned  by  looking  out  the 
names  in  a  mythological  dictionary.     Maps  and  globes 
of  the  heavens  once  had  the  figures  of  these  imaginary 
beings  painted  on  them,  but  they  were  confusing  to  users 
of  maps,  so  they  are  now  omitted.     The  simple  maps 
attached  to  this  book  give  only  the  brighter  stars ;  but, 
on  maps  giving  a  full  representation  of  the  stars,  the 
constellations  are  bounded  by  dotted  lines,  like  political 
divisions  on  a  map  or  globe  of  the  earth. 

3.  Magnitudes. — The  fixed  stars  are  classified  accord- 
ing to  their  apparent   size  or  brightness.      Beginning 
with  the  brightest,  they  are  called  stars  of   the  first, 
second,  and  third   magnitudes,  etc.     In  this  book  the 
abbreviations  ist  m.,  2d  in.,  3d  m.,  etc.,  are  used.     There 
are  six  magnitudes  of  stars  visible  to  the  naked  eye. 
The  stars,  however,  differ  gradually  in  brightness,  so 


1  The  small  deviation  from  the  truth  of  this  is  given  afterward. 
2 


that  the  observer  must  not  expect  to  see  the  magnitudes 
very  distinct. 

4.  Nomenclature. — The  ancients  gave  names  to  the 
brighter  stars,  and   these  are   generally  retained  ;   but 
astronomers  now  use,  as  symbols  to  distinguish  stars, 
the  letters  of  the  Greek  alphabet,  to  which  is  added  the 
genitive  of  the  Latin  name  for  the  constellation.     Thus, 
a  Aurigze,  or  Alpha  Aurigae,  indicates  the  brightest  star 
in  Auriga,  and  yS  Orionis  the  second  brightest  star  in 
Orion.     The  letters  are  used  as  symbols,  in  the  order  of 
magnitude  in  each  constellation. 

5.  ist  M.  Stars.— Fourteen  ist  m.  stars  are  visible  all 
over  the  United  States.     Their  names,  and  the  names  of 
the  constellations  containing  them,  are  given  below  : 
Arcturus  in  Bootes.  Capella  in  Auriga. 
Antares  in  Scorpio.  Pollux  in  Gemini. 
Altair  in  Aquila.  Procyon  in  Canis  Minor. 
Aldebaran  in  Taurus.  Sirius  in  Canis  Major. 
Betelguese  in  Orion.  Spica  in  Virgo. 

Rigel  in  Orion.  Regulus  in  Leo. 

Fomalhaut  in  Piscis  Australis.  Vega  in  Lyra. 

Besides  these,  another  ist  m.  star,  Canopus,  is  seen  in 
States  in  the  latitude  of  Tennessee,  and  farther  South. 
Over  forty  2d  m.  stars  are  seen  in  the  United  States. 

6.  Milky-Way. — In  studying  the  heavens  in  nature, 
the  student  must  observe  the  Milky-Way.    This  is  a  band 
or  zone  of  faint  cloudy  light,  stretching  round  the  heav- 
ens, and  inclosing  the  earth  like  a  ring.     By  watching  it, 
the  student  will  see  that  it  divides  at  one  place  into  two 
parts,  or  bands,  which  lie  side  by  side,  and  come  together 
again.     It  separates  the  heavens  into  nearly  equal  parts. 
A  small  portion  of  the  Milky-Way  is  too  near  the  south 
pole  to  be  seen  in  the  United  States. 

(See  Appendix  A,  for  reference  in  studying  the  con- 
stellations.) 

Greek  Alphabet. 


a  Alpha 
/3  Beta 

7  Gamma 

8  Delta 

e   Epsilon 
£  Zeta 


77  Eta 

0  Theta 

1  Iota 

K  Kappa 
X  Lambda 
/i  Mu 


v  Nu 
f  Xi 

o  Omicron 
•TT  Pi 
p  Rho 
7?  Sigma 


T  Tau 
v  Upsilon 
<J>  Phi 


01  Omega 


'M pi  :  V:  •I<.: 


ASTRONOMY  BY  OBSERVATION. 


CHAPTER    II. 

THE    DIURNAL   MOTION   OF  THE   STARS,    AND   HOW  TO 
OBSERVE   IT— HOW   TO   INTERPRET   IT. 

7.  Polar  Constellations. — Fronting  this  page,  there  is 
a  small  map  containing  three  very  important  groups  of 
stars,  called  the  Great  Dipper,  the  Little  Dipper,  and 
Cassiopeia's  Chair.  All  these  can  be  seen  by  an  ob- 
server in  Canada  or  any  part  of  the  United  States  on 
any  clear,  moonless  night,  at  dark,  with  the  following 
exceptions :  In  the  Southern  States  the  Great  Dipper  is 
so  low  down  in  midwinter  (being  partly  on  the  horizon), 
and  Cassiopeia  is  so  low  in  midsummer,  that  at  those 
seasons  it  is  not  possible  to  distinguish  them  very  clearly. 
But  even  in  midwinter  or  midsummer  the  observer  in 
the  Southern  States  can  easily  see  them  all  by  sitting  up 
a  little  later. 

In  order  to  observe  well  the  diurnal  motion  of  the 
stars,  it  is  necessary  to  know  these  groups.  It  is  best 
to  begin  with  the  Great  Dipper.  By  turning  the  map 
so  that  the  name  of  the  current  month  will  be  at  the 
top,  the  position  of  the  group  on  the  sky  at  eight 
o'clock  will  be  represented.  If  the  student  will  at  night 
attentively  consider  the  figure  of  the  Great  Dipper,  and 
the  part  of  the  map  upon  which  it  is  situated,  he  can  go 
right  out  of  doors  to  some  point  where  he  can  have  an 
unobstructed  view  toward  the  north,  and  at  once  find 
it.  By  drawing  the  constellation,  as  recommended  in 
the  Introduction  to  this  book,  his  acquaintance  with  the 
Great  Dipper  will  be  fixed. 

Next,  the  student,  holding  the  map  as  before  de- 
scribed, studies  attentively,  and  draws,  if  possible,  the 
figure  of  the  Little  Dipper.  There  is,  he  sees,  a  bright 
star  (named  Polaris)  in  the  handle  end.  There  are  only 
two  other  bright  stars,  and  they  are  in  the  bowl  of  the 
Little  Dipper.  They  are  called  the  "  Guardians  of  the 
Pole."  A  chain  of  faint  stars,  perfectly  distinct  on  a 
clear,  moonless  night,  forms  the  handle  and  part  of  the 
bowl  nearest  the  handle.  The  student  notes  the  peculiar 
curve  of  the  handle,  quite  different  from  the  handle  of 
the  Great  Dipper,  which  bends  backward. 

In  learning  the  Little  Dipper,  it  is  best  to  find  Polaris 
first.  It  will  aid  in  identifying  Polaris  if  the  student 
notes  that  there  are,  in  the  bowl  of  the  Great  Dipper, 
two  stars  which  lie  in  a  line  with  Polaris,  and  which  are 
therefore  called  "  The  Pointers."  After  discovering  Po- 
laris, it  is  best  for  the  student  always  to  find  next  the 
two  bright  stars  in  the  bowl  of  the  Little  Dipper,  called 
the  "  Guardians  of  the  Pole."  They  are  easily  found, 
because  no  other  two  stars  so  bright  and  so  near  each 
other  can  be  seen  so  near  Polaris.  The  fainter  stars  in 
the  Little  Dipper  all  lie.  as  the  student  may  see  from  the 


map,  between  Polaris  and  the  Guardians.  When  the 
night  is  not  very  clear,  they  are  not  easy  to  find,  but,  by 
looking  for  Polaris  first,  and  then  the  Guardians,  the 
fainter  stars  can  easily  be  discovered  if  visible. 

After  the  Little  Dipper  is  identified,  the  student 
studies  Cassiopeia's  Chair,  and  draws  it  if  possible.  He 
must  hold  the  paper  as  before,  and  regard  carefully  the 
shape  of  the  star-group  and  its  place  on  the  map.  The 
two  star-groups,  Cassiopeia's  Chair  and  the  Great  Dip- 
per, are  very  nearly  on  opposite  sides  of  the  Little  Dip- 
per. All  the  stars  of  the  Chair  are  bright,  except  the 
one  in  the  front  of  the  seat.  Without  this  faint  star,  the 
group  looks  like  a  very  irregular  W.  After  observing 
all  these  things  upon  the  map,  the  student  will  rarely 
find  any  difficulty  in  recognizing  the  "  Chair"  as  soon  as 
he  goes  out  of  doors  and  faces  north. 

8.  Diurnal  Motion  of  Polar  Constellations.  —  After 
making  acquaintance  with  these  groups,  the  student  is 
ready  to  undertake  the  principal  object  of  investigation 
in  this  chapter,  viz.,  the  motion  of  these  stars  for  twenty- 
four  hours,  or  their  diurnal  motion.  At  dark  the  map 
must  be  taken  from  the  book,  and  fastened  to  the  wall 
or  a  board  by  a  small  round  screw  or  pin  pierced  through 
Polaris,  and  permitting  the  map  to  revolve  round  the 
screw,  which  represents  Polaris.  The  map  is  then  turned 
upon  the  pin  so  that  the  positions  of  the  star-groups  re- 
semble those  in  the  heavens  at  the  earliest  hour  in  the 
evening  at  which  they  are  visible.  In  turning  the  map 
to  get  the  correct  aspect,  the  student  must  not  be  gov- 
erned wholly  by  the  name  of  the  month  at  the  top,  but 
he  must  make  the  appearance  correspond  exactly  with 
the  look  of  the  heavens  at  the  time. 

He  must  leave  the  map  in  this  position  on  the  wall, 
and  must  return  as  nearly  at  midnight  as  possible.  He 
will  find  that  the  star-groups  have  moved,  so  that  the 
map  no  longer  represents  their  positions  on  the  sky. 
But,  after  watching  the  new  arrangement  attentively,  he 
will  see  that,  by  revolving  the  map  a  little  on  the  screw 
(in  a  direction  contrary  to  that  in  which  watch-hands 
revolve),  he  can  again  make  it  represent  the  situation  of 
the  star-groups  on  the  sky.  In  doing  this,  the  student 
must  not  perform  careless  and  hasty  revolving,  making  it 
necessary  to  correct  his  work  by  turning  the  map  back- 
ward on  the  screw.  He  must  be  slow  and  careful,  so 
that  it  will  not  be  necessary  to  revolve  the  map  in  any 
way  except  that  contrary  to  the  watch-hand  motion. 

If  the  student  leaves  the  map  as  then  rectified,  and 
rises  in  the  morning  just  before  light,  to  take  another 
look  at  the  stars  in  the  north,  he  will  find  that  the  star- 
groups  have  again  moved,  so  that  the  map  must  be  cor- 
rected. ..But  a  little  further  revolution  in  the  same  di- 
rection will  again  set  it  right. 


\co 


THE  DIURNAL  MOTION  OF   THE  STARS:   HOW   TO   OBSERVE  IT— HOW  TO  INTERPRET  IT.    II 


Again,  on  the  next  evening  at  dark,  the  map  is  found 
to  need  turning.  The  student  will  find  that,  by  com- 
pleting the  revolution  before  partially  made,  the  map 
will  again  represent  the  appearance  of  the  star-groups 
in  the  heavens. 

If,  in  addition  to  this  work,  a  student  selects  any 
group  of  stars  which  he  sees  over  the  eastern  horizon  at 
dark,  and  which  he  thinks  he  could  identify,  if  at  dark 
he  notes  them  well,  to  make  sure  that  he  can  remember 
them,  and  if  he  then  comes  out  again  to  look  for  them 
at  or  near  midnight,  he  will  see  that  they  have  moved 
from  their  earlier  position,  and  are  on  the  zenith,  or  near 
it.  If  he  rises  to  look  again  before  day,  he  will  probably 
find  that  the  star-group  has  disappeared,  and,  as  he  will 
probably  think,  below  the  horizon  in  the  west.  On  the 
next  evening  at  dark  he  will  discover  this  cluster  in  the 
same  place  (so  far  as  he  can  judge)  in  which  he  first  saw 
it,  viz.,  just  above  the  eastern  horizon. 

9.  From  such  observations  as  these  we  learn  that  all 
the  stars  in  the  heavens  appear  to  revolve  round  the 
earth  from  east  to  west.    In  the  heavens  Polaris  is  nearly 
at  the  center  of  motion,  and  does  not  seem  to  move  at 
all,  while  all  the  others  circle  round  it.     The  position  of 
Polaris  resembles  that  of  the  points  at  the  poles  of  the 
earth.     Polaris  is  called  the  pole-star. 

The  student  is  probably  already  quite  familiar  with 
the  fact  that  the  sun  has  this  same  appearance  of  revolv- 
ing round  the  earth.  His  presence  prevents  us  from 
seeing  stars  in  the  daytime.  Men  who  go  down  in  deep 
wells  and  mines,  where  their  eyes  are  not  blinded  by  the 
sun's  dazzling  light,  report  that  they  see  stars  in  the  sky 
through  the  narrow  opening. 

10.  Motions  on  Earth. — Now,  the  student  has  prob- 
ably often  noticed,  when  riding  in  a  carriage,  that  trees 
and  other  bodies,  which  he  knows  to  be  at  rest,  seemed 
to  move  in  consequence  of  his  own  motion.     But,  more 
than  this,  almost  every  student  has  been  so  situated  on  a 
few  occasions  that  his  senses  had  a  very  strong  delusive 
impression  that  other  objects  were  moving,  when  yet  his 
reason  convinced  him  that  they  were  at  rest  and  he  was 
himself  in  motion.     A  person  in  a  boat  pushing  off  from 
shore  sometimes  feels  this  delusion  very  strongly.     In 
all  such  cases  the  mover  is  carried  without  any  conscious 
exertion,   without   any    perceptible   jarring,   jolting,  or 
noise.     If,  now,  the  earth  revolved  without  affecting  any 
of  our  senses  except  sight,  it  is  certain  we  should  receive 
a  very  strong  delusive   impression   that  sun  and  stars 
were  moving,  and  we  ourselves  were  at  rest.     But  there 
is  not  anything  in  all  these  facts  to  decide  for  us  whether 
the  earth  moves  or  whether  sun  and  stars  move.     We 
only  know  that  the  stars  would  appear  to  move  whether 
they  move  or  we ;   and,  without   some   further  experi- 


ence, we  should  not  be  able  to  determine  which  was  the 
mover. 

11.  Plane  of  the  Motion. — We  call  the  direction  in 
which  the  earth  moves  east ;  that  in  which  the  stars 
move,  or  seem  to  move,  west.     If  the  student  will  stick 
pins  in  parts  of  his  dress  at  various  heights  from  the 
floor,  and  will  then,  standing  on  one  spot,  revolve  *  axial- 
ly,  or  turn  round,  he  will  notice  that  all  the  pins  revolve  ; 
that  a  line  drawn  through  the  centers  of  all  these  revo- 
lutions would  be  a  vertical  line,  or  a  line  perpendicular 
to  the  floor.     The  pins  move  in  circles,  since  throughout 
their  motion  they  are,  each  one,  all  the  time  at  the  same 
distance  from  the  center  or  axis  (as  the  vertical  line  is 
called),  and  each  pin  is,  at  every  point  of   its  revolu- 
tion, at  the  same  distance  from  the  floor.     Therefore  the 
circles  are  all  parallel  to  each  other  and  the  floor,  and 
perpendicular  to  the  axis.     Every  particle  in  the  stu- 
dent's body  revolves  in  circles  parallel  to  the  floor  and 
perpendicular  to  the  axis.     The  earth's  rotation  on  her 
axis  in  twenty-four  hours  makes  every  particle  of  matter 
in  it  move  in  a  circle.     Some  of  the  circles  are  larger, 
some  smaller ;  and  all  the  centers  are  on  a  line  called 
the  axis,  which  is  perpendicular  to  the  planes  of  the 
circles. 

12.  East  and  West. — Now,  all  these  particles  move 
in  parallel  lines  in  the  same  direction.     We  call  that 
direction  east.     West  is  the  opposite  direction,  or  that 
in  which  sun  and  stars  seem  to  move  daily  around  the 
earth.     East  and  west  are  not  directions  in  the  sense  in 
which  north  and  south  are.     If  a  traveler  could  move 
round  the  earth  through  the  poles,  we  should  call  the 
direction  of  his  motion  north  until  he  reached  the  north 
pole,  and  after  that  we  should  call  it  south.     But  if  he 
journeys  round  the  earth  on  the  equator  or  on  any  of  its 
parallels,  we  call  the  direction  of  his  whole  motion  east, 
or  else  we  call  the  whole  west.     The  explanation  of  this 
is  that  east  and  west  are  mere  directions  of  revolution, 
while  north  and  south  are  fixed,  or  absolute  directions. 

When  we  say  "  The  sun  rises,"  the  people  on  the  other 
side  of  the  globe  say  "  The  sun  sets."  When  both  we  and 
they  face  north,  the  sun  is  on  our  right,  on  their  left,  at 
the  period  which  we  call  sunrise.  This  follows  from 
the  fact  that  their  heels  and  our  heads  turn  in  the  same 
direction. 

13.  The  Poles. — There  are,  of  course,  two  centers  of 
motion,  or  poles,  on  the  celestial  sphere.    Travelers  who 
have  been  south  of  the  equator  tell  us  there  is  no  star 
near  enough  to  the  center  to  be  actually  motionless  ;  but 
they  can  see  a  part  of  the  heavens  around  which  the 
stars  move  in  smaller  and  smaller  circles. 

*  This  must  actually  be  done. 


12 


ASTRONOMY  BY  OBSERVATION. 


14.  Polaris. — Travelers  also  tell  us  that,  at  the  equa- 
tor, Polaris  is  on  the  horizon  ;  and  as  they  journey  north 
it  rises  above  the  horizon,  its  distance  from  the  north 
point  of  the  horizon  being  always  equal  in  degrees  to 
the  observer's  distance  from  the  equator  of  the  earth,  or, 
in  other  words,  to  his  terrestrial  latitude.     Thus,  sailors 
at  sea  ascertain  their  latitude  by  measuring  the  height 
of  the  pole-star  above  the  horizon.     At  the  pole  Polaris 
would  be  on  the  zenith.     Any  student  of  this  book  who 
chances  to  journey  far  north  or  south  must  be  sure  to 
look  at  the  pole-star. 

15.  Polaris  is  not  exactly  at  the  center  of  motion.     It 
is  more  than  a  degree  from  the  pole,  and  so  moves  in  a 
very  small  circle  round  it.     It  is  so  near  that  with  ordi- 
nary observation  it  appears  to  be  at  the  pole ;  but  by 
very  careful  watching  any  student  may  detect  the  motion 
of  Polaris.    This  is  most  easily  discovered  when  its  posi- 
tions at  dark  in  the  evening  and  just  before  light  in  the 
morning  lie  on  a  vertical  or  on  a  horizontal  line.     We 
can  tell  when  this  is  the  case  by  the  fact  that  the  pole, 
Polaris,  and  the  star  Caph  in  Cassiopeia's  Chair,  are  all 
nearly  in  line.     Caph  is  the  star  at  the  lower  end  of  the 
front  leg  of   the  Chair.     When    Polaris  and   Caph  are 
in  a  vertical  line  in  the  early  evening,  the  early  evening 
and  early  morning  positions  of  Polaris  lie  on  a  nearly 
vertical  line.     The  same  rule  holds  good  when  the  line 
is  horizontal.     The  variation  is  horizontal  in  September 
and  March ;  vertical  in  December  and  June.     At  other 
times  it  is  oblique  and  not  so  easy  to  detect.     When  it 
is  horizontal,  the  student  finds  at  dark  a  place  where  he 
can  stand  and  see  Polaris  just  on  or  behind  a  vertical 
line  formed  by  a  side  of  some  house,  window,  or  other 
object.     If  the  variation  is  vertical,  the  line  of  the  house, 
etc.,  must  be  horizontal.     In  the  morning,  while  it  is  yet 
dark,  the  student  must  stand  exactly  where  he  stood  be- 
fore, and  he  will  find  that  Polaris  no  longer  touches  the 
line.     The  star  will  have  moved  behind  the  house  or 
farther  from  it. 

16.  The  Celestial  Sphere.*— When  we  look  at  the 
heavens  on  a  starry  night,  we  seem  to  see  half  of  a  great 
globe  or  sphere  on  which  appear  thousands  of  stars. 
This  look  of  a  globe  is  delusive,  and,  when  stars  seem  to 
be  side  by  side  on  the  sphere,  it  is  only  because  we  see 
them  in  nearly  the  same  direction  from  ourselves,  and 
thus  we  project  them  on  neighboring  points  of  the  im- 
aginary sphere  in  the  center  of  which  we  seem  to  be 
situated.     Thus,  in  Fig.  i,  the  observer  at  Csees  the  ob- 

*  This  subject  is  treated  after  that  of  the  diurnal  motion  in  order  that  the 
knowledge  of  the  motion  of  the  stars  round  the  pole  may  aid  the  student  in 
attaching  ideas  of  reality  to  the  work.  The  account  is  very  short  and  simple, 
because  a  detailed  statement  would  convey  no  ideas  to  those  who  use  no  in- 
strument, and  make  no  measurements  on  the  sphere. 


jects  at  a  and  b,  projected  near  each  other  on  the  sphere, 
though  they  are  really  far  apart. 

17.  This    appearance 
of  a  sphere  makes  it  con- 
venient for  us  to  repre- 
sent the  stars  on  a  globe, 
in  order  to  estimate  dis- 
tances.     It   is   true,  the 
heavens  are  concave,  and 
the  globe  is  convex.   But, 
let  us  suppose  the  globe 
were  made  of  very  thin 
blue  glass,  and  the  stars 
were  painted  inside  so  as 
to  show  through.     It  is 

evident  the  proportional  distances  would  be  the  same  on 
the  concave  and  convex  sides.  We  need  an  artificial 
globe,  mainly  to  show  circles  and  estimate  distances — 
not,  as  in  the  case  of  the  terrestrial  globe,  to  show  us  how 
the  sphere  looks. 

18.  The  Circles. — All  students  of  geography  will  re- 
member how  we  take  the  measurements  called  latitude 
and  longitude  by  the  aid  of  the  opposite  points  called 
poles,  and  by  the  great  circle  lying  half-way  between 
them  called   the  equator.     The  other  circles  used  are 
parallels  to  the  equator,  and  perpendiculars  to  it  pass- 
ing through  the  poles. 

On-  the  celestial  sphere  also,  the  centers  of  motion  are 
used  as  poles,  and  a  great  circle  called  the  equinoctial 
is  measured  half-way  between  the  poles.  The  parallels 
are  called  diurnal  circles,  and  the  great  circles  running 
through  the  poles  are  called  hour-circles.  Distances 
are  measured  east  and  west,  north  and  south,  just  as  we 
measure  latitude  and  longitude ;  but  they  are  called  in- 
stead declination  and  right  ascension.  Very  often  they 
are  designated  by  the  symbols  R.  A.  and  D.* 

The  earth's  axis  is  a  part  of  the  line  joining  the  celes- 
tial poles,  the  plane  of  the  equator  is  the  plane  of  the 
equinoctial,  and  thus  these  ways  of  measuring  the  earth 
and  the  heavens  correspond  exactly. 

But  it  is  puzzling  to  young  students  that,  whereas 
there  is  only  one  set  of  circles  to  measure  the  earth, 
three  are  used  in  measuring  the  heavens.  At  present, 
the  student  will  be  expected  to  hear  of  only  one  more. 

19.  The  terrestrial  circle  which  we  call  the  horizon 
of  a  place  does  not  divide  the  earth  in  halves,  and  so  is 
not  a  great  circle  ;  but  it  outlines  a  circle  on  the  celestial 
sphere  called  the  celestial  horizon,  which  divides  that 
sphere  into  halves,  the  visible  and  the  invisible  halves. 

*  The  student  will  more  easily  remember  that  R.  A.  corresponds  to  longi- 
tude by  recollecting  that  stars  in  moving  from  east  to  west  ascend  on  our 
right.  ( 'J  lie  observer  facing  north) 


THE  DIURNAL  MOTION  OF   THE  STARS:  HOW   TO  OBSERVE  IT— HOW   TO  INTERPRET  IT.    13 


Therefore  the  celestial  horizon  is  a  great  circle,  and  the 
point  of  the  sphere  over  our  heads,  called  the  zenith,  and 
that  under  our  feet,  called  the  nadir,  are  its  poles.  There 
are  supposed  to  be  circles  parallel  to  the  horizon,  and 
others,  called  vertical  circles,  running  through  its  poles. 
The  distances  measured  on  the  horizon  system  of  circles 
are  vertical  distances,  called  altitude,  or  horizontal  dis- 
tances, called  azimuth. 

The  horizon  system  of  circles  seems  to  us  fixed  in 
space,  while  the  equinoctial  system  seems  to  revolve. 
But  no  two  different  places  on  earth  have  the  same  -part 
of  the  heavens  above  their  horizons  at  the  same  time ; 
and,  therefore,  the  measurements  D.  and  R.  A.,  which 
are  the  same  all  over  the  world,  have  wider,  more  abid- 
ing value  than  altitude  and  azimuth. 

There  is  a  very  important  circle  belonging  to  the 
horizon  system  called  "the  meridian."  It  differs  for 
places  differing  in  terrestrial  longitude,  but,  when  peo- 
ple speak  of  "  the  meridian,"  it  is  generally  under- 
stood that  they  refer  to  the  meridian  *  of  the  place  in 
which  they  live,  or  of  which  they  are  talking.  The 
meridian  runs  through  the  zenith  and  nadir,  through 
the  north  and  south  celestial  poles,  and  also  through 
what  are  called  the  north  and  south  points  of  the  hori- 
zon. These  are  the  points  of  the  horizon  which  would 
be  reached  by  a  line  running  north  and  south  from  the 
observer.  The  meridian  divides  the  heavens  into  east- 
ern and  western  hemispheres  ;  but,  since  the  sphere  is  all 
the  time  revolving,  the  eastern  and  western  hemispheres 
change  in  area  continually. 

20.  The  distance  between  the  celestial  poles  equals 
1 80°.  The  distance  between  the  north  and  south  points 

FIG.  2. 


of  the  horizon  also  equals  180°.    In  Fig.  2  the  circle  rep- 
resents the  meridian.     If,  from  the  arc  of  the  circle  in 

*  The  student  should  point  to  the  meridian  by  revolving  his  finger.     He 
should  also  indicate  with  his  finger  the  various  points  on  it. 


Fig.  2,  running  from  the  north  point  through  the  south 
point  to  the  south  pole,  we  subtract  the  distance  between 
the  poles,  or  180°,  we  get  for  remainder  the  distance  from 
the  north  pole  to  the  horizon.  If  from  the  same  arc  we 
subtract  the  distance  from  the  north  point  to  the  south 
point,  also  180°,  we  get  for  remainder  the  distance  of  the 
south  pole  from  the  horizon.  The  remainders  must  be 
equal,  since  we  subtract  equals  from  the  same. 

The  north  pole  is,  therefore,  just  as  far  above  the 
horizon  of  any  place  as  the  south  pole  is  below  it.  Both 
distances  equal,  in  degrees,  the  observer's  distance  from 
the  earth's  equator ;  or,  in  other  words,  his  terrestrial 
latitude. 

21.  If  the  north  point  of  the   horizon   left  a  visible 
mark  on  all  the  points  of  the  celestial  sphere  which  pass 
through  it,  it  would  trace  a  circle  round  the  north  pole 
of  the  heavens,  since  these  points  must  all  be  at  the 
same  distance  from  the  pole.     The  area  within  this  cir- 
cle is  always  visible  to  us,  and  therefore  it  is  called  the 
Circle  of  Perpetual  Apparition.     A  similar  circle  would 
be  traced  by  the  south  point  around  the  south  pole ; 
and,  as  we  never  see  the  part  of  the  heavens  within  this 
circle,  it  is  called  the  Circle  of  Perpetual  Disparition. 
The  radius  of  these  circles  is  equal  to  the  observer's  lati- 
tude on  earth.     As  we  travel  north,  these  circles  grow 
larger,  since  the  distance  of  the  north  point  from  the 
pole  star  increases.     At  the  north  pole  Polaris  is  on  the 
zenith  (nearly),  and  the  two  circles  are  each  a  hemisphere. 
The   pole   above  the    horizon  is   called    "  the   elevated 
pole  " ;  that  below  the  horizon,  "  the  depressed  pole." 

22.  The  Paths  of  Stars  over  the  Sky.— The  student 
is  strongly  recommended  to  study,  by  observation,  the  diurnal 
circles  in  which  stars  seem  to  move  over  the  sky.     He  must  see 
the   position  of  stars  or  star-groups,  first,  when   they  appear 
above  the  horizon,  and  afterward,  when  they  are  seen  on  or 
near  the  meridian.     He  will  find  that  when  they  are  on  the 
meridian  they  seem  to  be  farther  south  than  at  their  rising  or 
at  their  setting.    But  this  is  not  equally  true  of  all  stars.    Those 
that  rise  far  in  the  northeast,  like  Vega  in  the  constellation 
Lyra,  and  Capella  in  the  constellation  Auriga,  seem  on  reach- 
ing the  meridian  to  have  moved  much  farther  south  than  those 
which  rise  in  the  east.     Finally,  the  path  of  those  rising  in  the 
southeast  scarcely  seems  to  curve  southward  at  all. 

This  southward  direction  of  the  curve  is  apparent,  not  real, 
and  the  cause  of  the  appearance  is  the  elevation  of  one  pole 
above  the  horizon,  the  depression  of  the  other  below  it.  This 
is  evident  from  Fig.  3.  This  circle  shows  the  eastern  hemi- 
sphere of  the  heavens  about  as  it  appears  in  the  larger  part  of 
the  United  States. 

From  this  same  figure  it  is  evident  that  the  circles  of  the 
diurnal  motion  of  the  stars  are  very  unequally  divided  by  the 
horizon.  This  inequality  increases  as  the  circles  lie  farther 
north.  The  stars  which  have  a  smaller  part  of  their  diurnal 


ASTRONOMY  BY  OBSERVATION. 


circles  above  the  horizon  seem  to  move  in  a  less  curved  path 
than  those  a  great  part  of  whose  diurnal  circles  lies  above  the 

FIG.  4. 


horizon.     Fig.  4  gives  the  appearance  of  these  circles  on  the 
hemisphere  above  the  horizon. 

23.  Fig.  5  shows  the  eastern  hemisphere  of  the  heav- 


FIG.  5. 

z 


CE 


ES 


IAL 


O 

C 

zHQ 


RIZ 


ON 


a.  POLE 


Fiu.  6. 


ens  to  an  observer  at  the  equator.  The  poles  are  on  the 
horizon,  and  the  equinoctial  runs  through  zenith  and 
nadir.  At  the  equator,  the  circles  in  which  the  stars 
revolve  seem  to  run  directly  east  and  west,  with  no  ap- 
pearance of  curv- 
ing north  or  south. 
The  circle,  Fig.  6, 
exhibits  the  hemi- 
sphere above  the 
horizon  at  the  north 
pole.  There  the 
poles  of  the  heav- 
ens are  at  the  ze- 
nith and  the  nadir, 
and  the'  horizon  co- 
incides with  the 
equinoctial.  The 
diurnal  circles  of 
the  stars  lie  whol- 
ly above  or  wholly 
below  the  horizon.  At  the  north  pole  no  stars  are  vis- 
ible but  those  north  of  the  equinoctial,  and  they  move 
in  circles  parallel  to  the  horizon. 

South  of  the  equator  corresponding  conditions  exist. 


CHAPTER    III. 

ANNUAL    MOTIONS    OF    THE    STARS    AND    SUN,    AND    HOW 
TO    OBSERVE    THEM— HOW    TO    INTERPRET    THEM. 

24.  The  Annual  Motion  of  the  Stars. — When  we 
observe  the  diurnal  motion  of  the  stars,  we  take  all  our 
observations  on  the  same  night,  at  intervals  of  one,  two, 
or  three  hours ;  but,  when  we  observe  the  annual  mo- 
tion of  the  stars,  we  make  our  observations  at  intervals 
of  two  or  three  weeks,  and  it  is  always  necessary  to 
make  them  at  the  same  hour  of  the  night,  since  we  thus 
avoid  any  confusion  from  seeing  also  the  diurnal  motion 
of  the  stars. 

In  order  to  observe  the  annual  motion  it  is  necessary 
to  know  at  least  one  star-group,  so  that  we  can  identify 
it  if  it  changes  place.  If  we  know  but  one,  it  should  be 
just  above  the  eastern  horizon  at  the  hour  for  observa- 
tion ;  and,  also,  whether  the  observer  knows  one  or 
many,  it  is  best  for  him  to  make  special  observation  of 
one  situated  in  that  place  when  he  begins.  If,  after  see- 
ing it,  he  waits  two  weeks  and  takes  another  look,  he 
will  find  that  it  is  no  longer  so  near  the  eastern  horizon ; 
it  is  higher  up,  or  farther  west.  If  he  looks  at  it  again, 
after  an  interval  of  two  or  three  more  weeks,  he  will 
find  that  it  seems  to  have  continued  its  motion  west.  If, 


ANNUAL  MOTIONS  OF   THE  STARS  AND  SUN. 


during  six  months,  he  keeps  on  observing  it,  at  intervals 
of  three  or  four  weeks,  it  will  appear  to  have  crossed 
the  evening  sky,  and  at  the  end  of  the  six  months  will 
have  passed  out  of  sight  below  the  western  horizon. 
After  about  a  month  he  will  be  able  to  see  it  in  the 
morning  sky  just  before  light.  It  will  not  be  far  from 
the  eastern  horizon,  and  it  will  be  evident  that  it  has 
passed  from  the  eastern  to  the  western  side  of  the  sun. 
If  he  continues,  at  intervals  of  a  few  weeks,  to  rise  before 
day  in  the  morning  and  note  its  position,  it  will  again 
appear  to  him  to  be  moving  west ;  and  in  another  six 
months  it  will  again  have  disappeared  below  the  western 
horizon.  After  this  it  can  be  found  on  the  evening  sky 
where  he  had  first  seen  it,  that  is,  just  above  the  eastern 
horizon.  Thus  it  would  appear  to  have  made  a  revolu- 
tion around  the  earth  in  a  year's  time. 

25.  Though  it  is  best  to  concentrate  special  observa- 
tion on  one  group,  the  observer  would  have  seen  that 
all  the  other  groups  he  knew  moved  in  exactly  the  same 
direction  and  at  the  same  rate  of  motion.    The  direction 
of  the  annual  motion  of  the  stars  is  precisely  like  the 
direction  of  their  diurnal  motion.     This  is  very  evident 
if  we  watch  the  stars  lying  near  the  north  pole  of  the 
heavens.      They   seem   to   make   an   annual    revolution 
round  the  pole  in  precisely  the  direction  in  which  they 
make  their  diurnal   revolution.      The  resemblance   be- 
tween the  diurnal  and  annual  motions  is  so  marked  that 
but  for  one  fact  we  should  conclude  that  they  had  the 
same  cause,  viz.,  the  earth's  rotation  on  her  axis./  But  in 
watching  the  diurnal  revolution  we  find  at  the  end  of  the 
intervals  of  observation  that  the  sun  must  have  moved 
with  the  stars,  since  an  hour  or  two  has  elapsed.     On 
the  other  hand,  in  watching  the  annual  motion  we  make 
observation  at  the  same  hour  (so  long  as  the  sun  is  visi- 
ble in  the  evening  or  morning  sky),  and  therefore,  after 
the  intervals,  the  sun  can  not  be  in  a  changed  position. 
Now,  we  know  from  experience  that  an  axial  rotation 
makes  the  sun  revolve  as  well  as  the  stars,  and  there- 
fore we  are  forced  to  think  that  the  annual  motion  of 
the  stars  must  be  due  to  some  other  cause.  ' 

26.  Solar  and  Sidereal  Days.— In  watching  the  diur- 
nal motion  of  the  stars,  as  described  in  the  last  chapter, 
we  usually  note  their  positions  both  at  the  beginning 
and  end  of  a  revolution ;  that  is,  at  the  same  hour  on 
two  successive  days.     Thus  observed,  the  stars  appear 
to  us  to  be  in  exactly  the  same  position  at  the  same 
hour  of  successive  days.     But,  after  the  observation  de- 
scribed in  this  chapter,  it  is  evident  that  this  conclusion 
is  not  strictly  accurate ;  but  the  stars  in  one  day  make 
so  little  more  than  a  revolution  that  we  do  not  perceive 
the  difference.     In  two  or  three  weeks  the  differences 
accumulate,  and  the  truth  becomes  evident  to  the  eye. 


(During  a  year  the  stars  make  one  revolution  more  than 
the  sun.! 

Let  us  suppose  that  at  midnight,  when  we  know  that 
the  sun  is  on  the  meridian  below  the  horizon,  we  see  a 
star  on  the  meridian  above  the  horizon.  At  the  next 
midnight  this  star  must  be  a  very,  very  little  west  of  the 
meridian.  Therefore  it  must  come  to  the  meridian  be- 
fore the  sun,  which  reaches  that  point  at  midnight.  If, 
now,  we  call  the  intervals  between  the  sun's  successive 
passages  over  the  meridian  below  the  horizon  a  Solar 
Day,  and  the  interval  between  the  star's  successive  pas- 
sages over  the  meridian  above  the  horizon  a  Sidereal 
Day,  it  is  clear  that  a  solar  day  is  a  very  little  longer 
than  a  sidereal  day. 

27.  The  student  who  merely  reads  about  this  annual 
motion  of  the  stars,  but  does  not  see  it  in  nature,  can 
not  have  a  practical  or  respectable  knowledge  of  astrono- 
my.    Systematic  observations  at  the  same  hour  for  two 
months,  showing  that  the  stars  move  west  while  the  sun 
does  not,  are  of  the  greatest  value.     The  full  revolution 
merely  repeats  the  result  of  the  few  weeks'  experiments  ; 
and,  after  them,  the  more  general  observations,  made  by 
any  one  who  looks  at  -the  heavens  on  starry  nights,  are 
sufficient  to  show  that  the  revolution  continues.     It  is, 
however,  well,  a  year  afterward,  to  take  a  special  look 
at  the  star-group  in  its  old  place  above  the  western  hori- 
zon. 

28.  The  Annual  Motion  of  the  Sun. — The  observation 
of  this  motion  may  be  begun  at  any  time,  but  it  will  be 
here  described  as  it  appears  to  a  person  who  begins  to 
watch  it  on  September  2ist,  near  the  commencement  of 
a  school  year. 

The  observer  must  at  sunset  go  to  some  convenient 
place  and  note  first  the  direction  from  himself  of  the  set- 
ting sun.  He  will  find  that  the  sun  is  due  west  of  him 
at  sunset  on  September  2ist.  He  then  notes,  so  that  he 
could  remember  and  identify  it,  the  exact  point  of  the 
horizon  behind  which  the  sun  sets.  If  he  observes  care- 
fully, he  will  see  some  inequality  at  that  point  of  the 
horizon  which  will  enable  him  to  fix  it  in  memory.  In 
two  or  three  days  he  returns  at  sunset  to  the  same  spot, 
and  again  notes  the  point  of  the  horizon  behind  which 
the  sun  disappears.  He  finds  the  sun  appears  to  have 
moved  south.  If  he  continues  to  return  and  note  the 
sun's  positions  at  intervals  of  a  few  days  or  a  week,  he 
will  find  that  it  keeps  on  moving  south  until  a  few  days 
before  December  22d.  For  several  days  before  and  after 
December  22d  it  seems  to  set  at  exactly  the  same  point 
of  the  horizon.  This  is  usually  expressed  by  saying  that 
the  sun  "  becomes  stationary "  about  December  22d. 
Some  days  after  that  date  it  is  again  seen  moving  upon 
the  horizon,  but  it  is  found  going  north.  The  sun,  after 


16 


ASTRONOMY  BY  OBSERVATION. 


this,  keeps  on  moving  northward,  and  on  March  2ist  is 
again  due  west  at  sunset.  If,  after  this,  the  observer  takes 
a  look  at  convenient  intervals,  he  will  see  the  sun  farther 
and  farther  north  until  a  few  days  before  and  after  June 
22d,  when  it  again  appears  stationary  on  the  horizon. 
The  observer  should  take  note  that  the  sun  is  northwest 
of  him  on  June  22d.  When  the  sun  again  begins  to 
move,  the  direction  of  the  motion  is  southward,  and  this 
continues  until  September  2ist,  when  the  sun  is  again 
due  west  at  sunset.  Thus  the  interval  of  a  year  finds 
the  sun  at  the  same  point  of  the  horizon  and  moving  in 
the  same  direction. 

29.  If,  in  the  manner  described  above,  we  watched 
the  sun  at  his  rising  above  the  eastern  horizon,  we  should 
see  precisely  the^same  phenomena  of  motion  north  and 
south,  taking  place  at  exactly  the  same  times.     If  the 
sun's  positions  be  noted  at  midday,  this  north  and  south 
motion  can  be  detected,  but  it  requires  a  much  longer 
interval  of  time  to  perceive  it,  since  we  have  no  visible 
line  like  the  horizon  to  measure  it.     If  we  look  at  the 
sun  (through  a  thin  silk  umbrella)*  on  December  22d, 
and  again  on  June  22d,  the  variation  in  position  from 
north  to  south  is  very  evident.     The  student  should  cer- 
tainly look  at  it  thus  at  midday  June  22d,  and  note  that 
it  is  even  then  not  quite  vertical.     It  is  never  vertical  in 
the  United  States.     On  June  22d  it  is  vertical  at  places 
on  the  Tropic  of  Cancer,  and  on  December  22d  at  places 
on  the  Tropic  of  Capricorn.     This  is  the  meaning  of 
those  circles. 

30.  Probably  many  students  have  some  vague  recol- 
lection of  something  like  the  motions  of  the  sun  here 
described.     These  very  indefinite  remembrances  are  not 
worth  much.     The  student  should  make  special  and  sys- 
tematic observations.     He  should  see  part  at  least  of  the 
motion  north  and  the  motion  south  ;  he  should  see  the 
sun  through  at  least  one  stationary  period ;    he  should 
see  it  on  either  March  2ist  or  September  2ist  when  due 
west;  and  he  should,  on  June  22d  and  December  22d, 
note  its  midday  positions  (through  an  umbrella).     The 
student  who  begins  observation   September   2ist,  and 
keeps  it  up  until  February,  gains  a  realizing  idea  of  the 
sun's  motions,  and  sees  all  that  is  important  except  its 
position  on  June  22d. 

But  those  persons  who  are  satisfied  with  reading 
about  this  motion  in  a  book,  without  making  any  ob- 
servations, can  have  no  satisfactory  or  even  respectable 
knowledge  of  astronomy. 

31.  The  Sun's  Motion  among  the  Stars. — We  do  not 
see  the  stars  when  we  see  the  sun,  and  we  attribute  this 
fact  to  his  blinding  brilliancy.     Our  reason,  aided  by 

*  But  the  student  must  be  very  careful  never  to  look  at  the  sun  at  midday 
with  unprotected  eyes.     It  is  in  the  highest  degree  dangerous. 


our  observation  of  them,  satisfies  us  that  they  must  be 
in  the  heavens  when  the  sun  is,  and  that,  if  we  could  see 
them  when  we  see  him,  we  should  see  him  against  the 
background  of  some  star-group  familiar  to  those  who 
know  the  heavens  well.  Also,  since  the  stars  and  sun 
have  each  a  different  motion,  it  is  clear  to  our  reason 
that  the  sun  would  not  always  be  in  the  same  constella- 
tion. If  the  sun,  like  a  meteor,  left  a  track  of  light  be- 
hind him,  it  would  trace  some  kind  of  path  among  the 
stars. 

32.  The  early  astronomers  long  ago  found  a  way  of 
tracing  among  the  stars  this  line,  or  path  of  the  sun. 
Their  process  for  it  is  easily  understood,  but  it  would 
divert  us  too  much  from  our  present  subject  to  describe 
it  here.     Astronomers  tell  us  that  if  we   could  see  the 
stars  when  we  see  the  sun,  we  should  always  see  him 
somewhere  on   a  great   circle   of   the   celestial   sphere 
passing  through  the  constellations  Aries,  Taurus,  Gem- 
ini, Cancer,  Leo,  Virgo,  Libra,  Scorpio,  Sagittarius,  Cap- 
ricornus,  Aquarius,  and    Pisces.     This  circle  is  called 
"  the  Ecliptic,"  and  it  is  traced  on  the  maps  attached  to 
this  volume.     If  we  could  see  the  sun  and  stars  together 
at  midday,  we  should,  in  the  course  of  a  year,  see  him 
successively  on  all  the  points  of  this  circle.     Since  the 
stars  move  west,  and  the  sun  sometimes  moves  north 
and   sometimes  south,   he   would   appear   to   fall  back, 
sometimes  northeast,  sometimes  southeast.     This  great 
circle  intersects  the  equinoctial  at  an  angle  of  23^°.     A 
zone  or  belt  of  the  heavens  extending  8°  on  each  side  of 
it  is  called  the  Zodiac  ;   and  so  the  constellations  Aries, 
Taurus,  etc.,  are  called  the  Zodiacal  Constellations. 

33.  Now,  although  the  student  is  indebted  to  astrono- 
mers for  tracing  this  circle  with  exactness,  and  for  tell- 
ing us  where  the  sun  is  in  it  on  any  day,  any  observer 
can  see  in  the  heavens  a  good  deal  to  confirm  their  testi- 
mony.    Unless  he  sees  these  things  for  himself,  what  he 
learns  from  books  will  not  be  a  knowledge  of  real  objects 
that  he  knows  by  sight,  and  thus  it  does  not  make  him 
look  at  the  phenomena  in  the  heavens  with  intelligence. 
In  the  first  place,  by  learning  the  zodiacal  constellations, 
he  sees  that  they  extend  in  a  ring  around  the  sphere. 
Besides  this,  he  will  always  find  one  of  them  at  dark  not 
far  from  the  point  where  the  sun  sets.     By  watching  it, 
the  observer  sees  where  it  sets,  and  he  will  also  perceive 
that  from  evening  to  evening  it  sets  earlier.     Finally,  it 
becomes  invisible ;  and   he  has  reason  to  think  this  is 
because  it  sets  nearly  when  the  sun  does,  and  thus  is 
obscured  by  his  light.     About  that  time  the  sun  sets 
where  it  was  seen  to  set.     Thus,  when  astronomers  show 
us  the  line  of  the  ecliptic  in  that  star-group,  and  tell  us 
exactly  when  the  sun  sets  at  each  point  of  it,  we  can  ac- 
cept their  statements  with  intelligence. 


ANNUAL  MOTIONS  OF    THE  STARS  AND   SUN. 


34.  But,  also,  astronomers  tell  us  that  a  line  drawn 
from  the  center  of  the  sun  through  the  center  of  the 
earth  would  always  end  in  a  point  of  the  ecliptic.    Thus 
we  are  always  between  the  sun  and  one  of  the  constella- 
tions of  the  zodiac.     We  can  roughly  test  this.     If  it  is 
true,  one  of  these  star-groups  must  always  be  on  the 
eastern  horizon  at  sunset,  and  in  the  same  direction  from 
us  that  we  are  from  the  sun.     That  is,  if  we  are  north- 
east from  the  sun,  it  must  be  northeast  from  us,  etc. 
This  constellation  must  be  just  above  the  eastern  hori- 
zon at  dark  when  the  sun  is  just  below  the  western  hori- 
zon.     Now,  we  always  find  one  of   the  zodiacal  star- 
groups  in  just  this  place,  and  in  such  a  direction  from  us 
that  we  seem  to  be  in  line  between  it  and  the  sun.    Also, 
if  we  sit  up  until  midnight,  when  the  sun  is  beneath  us, 
we  see   this  constellation   on  the    meridian  above   the 
horizon.* 

35.  The  Earth  the  Mover.f— Let  us  suppose  that,  on 
'June  22d,  the  student  began  to  note  the  constellations 

seen  above  the  eastern  horizon  at  dark.  In  June  Sagit- 
tarius would  be  there.  The  earth  would  be  between  the 
sun  and  Sagittarius.  If  occasional  observations  at  dark 
were  kept  up  until  December  22d,  the  student  would 
see  one  after  another,  just  above  the  eastern  horizon, 
Capricornus,  Aquarius,  Pisces,  Aries,  Taurus,  and  Gemi- 
ni. The  earth  would  be  between  the  sun  and  each  one 


NORTH-EAST  U 


in 


Positions  at  Sunset. 


succession.  From  the  eastern  horizon  they  would 
move  west,  and  on  December  22d  at  dark,  the  student 
would  see  them  all  (except  Sagittarius)  extending  in  line 
from  northeast  to  southwest,  or  to  where  the  sun  had 

*  This  observation  is  very  far  from  giving  exact  knowledge  ;  but  without  it 
the  study  of  astronomy  is  not  a  knowledge  of  real  objects  which  the  student 
can  see,  but  of  objects  which  for  the  student  exist  only  in  imagination. 

\  The  student  should  recite  this  section,  pointing  to  the  parts  of  the  heav- 
ens mentioned. 

3 


set.  Sagittarius  would  have  disappeared  nearly  a  month 
earlier,  and  the  student  who  had  watched  it  would  have 
reason  to  believe  that  it  was  far  southwest  of  the  sun. 

The  observer  would  remember  that  in  June  he  was 
between  Sagittarius  and  the  sun,  and  he  would  know  in 
December  that  Sagittarius  was  west  of  the  sun  at  dark. 
It  would  be  clear  that  something  had  moved  to  cause 
this  change  of  relative  position.  Remembering  what  he 
had  seen  during  six  months,  he  would  have  a  very  strong 
impression  that  the  sun  and  stars  had  moved. 

36.  Let  us  see  whether  a  motion  of  the  earth  could 
account  for  the  changes  of  position.    If  the  sun  and  stars 
did  not  move  from  June  to  December,  then  the  earth 
must  in  June  have  been  at  a  point  which  the  observer  at 
sunset  in  December  calls  "west  of  the  sun,"  and  the  earth 
must  have  moved  northeast  from   June  to  December, 
since  in  December  she  is  at  sunset  northeast  from  the  sun. 
Let  the  student  look  at  the  diagram  (Fig.  7).     If  the 
earth,  starting  from  s,  had  moved  through  all  the  posi- 
tions to  a,  she  would  have  passed  in  succession  between 
the  sun  and  Sagittarius,  Capricornus,  Aquarius,  Pisces, 
Aries,  Taurus,  and  Gemini.     She  would  also  have  made 
a  half-revolution  round  the  sun.     If  the  earth  moves,  she 
carries  the  observer  along  without  exertion  on  his  part, 
and   without   any   noise,   jolting,    or  jarring   whatever. 
Our  experience  of  motion  on  earth  shows  that  when  we 

are  thus  moved,  we  always 
get  the  impression  that  we 
are  at  rest,  and  that  sur- 
rounding objects  are  in  mo- 
tion. 

Thus,  the  supposition  that 
the  earth  is  the  mover  ac- 
counts for  the  apparent  mo- 
tion of  the  stars  just  as  well 
as  if  we  supposed  them  to 
be  the  real  movers.  It  ac- 
counts for  it  no  better,  as  far 
as  the  student  now  knows 
the  facts ;  but,  further  on,  the 
student  will  learn  some  facts 
which  can  be  much  better 
accounted  for  by  supposing 
that  the  earth  moves  than  by  supposing  that  she  is  at  rest. 

37.  Let  us  now  see  if  the  earth's  motion  will  account 
for  the  behavior  of  the  sun.     There  are  two  things  to 
explain :  First,  that  the  sun  does  not  appear  to  make  an 
annual  revolution  around  the  earth  with  the  stars.    Sec- 
ond, that  the  sun  moves  south  from  June  to  December. 
The  student  must  here  note  that  the  motion  of  the  sun 
north  and  south  is  a  motion  in  the  directions  of  the 
earth's  axis,  which  points  north  and  south. 


i8 


ASTRONOMY  BY  OBSERVATION. 


It  has  already  been  noticed  that,  if  the  earth  is  the 
mover,  she  must  ^rnove  northeast  from  June  to  Decem- 
ber ;  for,  when  we  see  the  heavens  at  dark  on  Decem- 
ber 22d,  we  see  plainly  that  her  December  position, 
between  Gemini  and  the  sun,  is  northeast  of  her  June 
position,  between  Sagittarius  and  the  sun.  From 
June  to  December,  the  sun's  annual  motion  is  from 
north  to  south.  Thus  the  earth,  if  she  is  the  mover, 
travels  north  while  the  sun  appears  to  travel  south, 

We  must  again  remember  our  experience  of  motion 
on  the  earth.  If  we  move  in  any  direction,  surrounding 
objects  seem  to  move  in  the  opposite  direction.  If  the 
motion  is  entirely  without  exertion  on  our  part,  without 
jolting,  jarring,  or  noise,  we  appear  to  ourselves  to  be  at 
rest  and  other  objects  seem  to  be  the  movers.  Thus  it 
seems  as  if  the  earth's  motion  might  be  the  cause  of  the 
sun's  apparent  motion  south  from  June  to  December. 

38.  But  since  the  earth's  supposed  motion  is  a  revo- 
lution round  the  sun,  this  matter  will  be  made  much 
clearer  by  some  experiments. 

Let  the  student  draw  a  circle  on  level  ground,  and 
get  an  assistant  of  just  his  own  height  to  stand  at  the 
center  while  he  walks  round  this  circle  to  the  right. 
His  head  moves  in  a  circle  round  his  assistant's  head  as 
a  center.  Let  the  walker  note :  (i)  That  an  object  out- 
side the  circle,  as  a  tree  for  example,  comes  into  view 
on  his  right,  gradually  gets  round  to  his  left,  disappears, 
and  finally  reappears  on  his  right.  Thus  this  object  ap- 
parently revolves  round  him  in  a  direction  contrary  to 
that  of  his  own  revolution  round  the  center,  but  in  the 
same  time.  Thus,  objects  outside  the  circle  appear  to 
revolve  round  the  walker.  (2)  The  object  at  the  center 
does  not  appear  to  revolve  round  the  walker.  Thus  it  is 
evident  that  the  theory  of  the  earth's  revolution  round 
the  sun  perfectly  accounts  for  the  fact  that  the  stars  have, 
the  sun  has  not,  an  apparent  annual  revolution  round  the 
earth. 

The  axial  revolution  was  omitted  in  this  experiment, 
to  avoid  confusion,  but  the  walker's  axis  is  a  vertical  line 
through  his  body,  and  its  directions  are  up  and  down. 
But  the  student  should  note  (3)  that  the  head  at  the  cen- 
ter does  not  move  up  and  down.  Therefore,  this  experi- 
ment does  not  account  for  the  sun's  motion  south  and 
north,  or  motion  in  the  directions  of  the  earth's  axis. 

39.  If,  now,  a  circle  is  drawn,  not  on  level  ground,  but 
on  some  rather  steep  hill-side,  crossed  by  a  road,  and 
thus  graded  into  an  inclined  plane,  the  observer  will  see 
a  variation  in  his  experiment.    As  he  ascends  the  hill,  the 
head  at  the  center  gets  lower  than  his  head  ;  and  as  he 
descends,  the  head  gets  higher.     These  are  motions  up 
and  down,  and,  therefore,  motions  in  the  direction  of  the 
axis,  which  is  vertical  in  this  case  as  in  the  other.     Of 


course  the  walker  is  under  no  illusion  that  the  head  at  the 
center  really  moves,  but  if  the  walker  were  moved  with- 
out exertion,  noise,  jolt,  or  jar,  there  would  be  illusion. 

When  the  experiment  is  performed  thus,  we  see  that 
we  can  account  for  the  sun's  north-south  motion,  if  we 
can  prove  that  the  earth's  revolution  is  similar  to  that  on 
the  hill-side.  Let  us  analyze  the  difference  in  the  experi- 
ments. In  both  cases  the  axis  is  vertical.  Where  the 
ground  is  level  and  the  plane  of  the  circle  is  horizontal, 
the  vertical  line  of  the  axis  is  perpendicular  to  the  plane 
of  the  circle.  But  where  the  circle  is  not  horizontal,  the 
vertical  line  of  the  axis  is  inclined  to  the  plane  of  the 
circle.  Almost  everybody  remembers  that  in  going  up- 
hill we  incline  toward  the  surface  of  the  ground  before 
us,  though  our  position  is  vertical.* 

Now  the  student  has  probably  already  been  told  that 
the  earth's  axis  is  inclined  to  the  plane  of  her  orbit.  The 
earth's  path  round  the  sun  is  called  her  orbit.  But  this 
statement  about  the  earth's  orbit  gives  no  real  and  prac- 
tical knowledge  of  astronomy,  unless  the  student  notes  the 
facts  before  his  eyes  on  the  heavens  which  prove  it. 

40.  If  he  knows  and  watches  the  zodiacal  constella- 
tions, as  described,  and  can  trace  the  ecliptic,  he  has 
some  real  idea  what  astronomers  mean  when  they  say 
that  the  earth  moves  round  the  sun,  between  the  sun 
and  the  ecliptic,  or  on  lines  extending  from  the  sun  to 
the  points  of  the  ecliptic.  A  circle  is  a  plane  figure,  and 
therefore  the  earth's  orbit,  or  path  round  the  sun,  is  in 
the  plane  of  the  ecliptic. 

The  celestial  axis  is  a  line  joining  the  celestial  poles. 
It  passes  through  the  center  of  the  ecliptic  and  all  great 
circles  of  the  celestial  sphere.  The  earth's  axis  also 
runs  north  and  south,  and  passes  through  the  earth's 
orbit  in  the  same  plane.  Two  lines  running  in  the  same 
direction  must  make  the  same  angle  with  the  same  plane. 
The  following  experiment  will  show  how  we  learn  that 
the  celestial  axis  is  not  perpendicular  to  the  plane  of  the 
ecliptic:  Let  the  student  cut,  from  rather  stiff  paper,  a 
circle  of  about  six  inches  in  diameter.  Let  him  insert, 
through  the  exact  center,  a  knitting-needle  or  straight 
straw.  Let  him  then  note  (i)  that  when  the  straw  is 
held  perpendicular  to  the  surface  of  the  paper,  the  end, 
or  extremity,  of  the  straw  is  at  equal  distances  from  all 
points  of  the  circumference  ;  (2)  when  the  straw  is  held 

*  These  and  similar  experiments  should  actually  be  performed  in  the  pres- 
ence of  the  teacher,  who  should  ask  questions,  making  sure  each  student  per- 
ceives the  exact  point  each  experiment  is  designed  to  show.  These  experi- 
ments are  not  striking,  as  are  many  which  are  utterly  useless  for  investigation, 
but,  if  the  teacher  can  set  his  pupils  to  thinking,  he  will  find  these  experiments 
better  than  mere  shows.  If  the  hill-side  experiment  must  be  imagined,  not 
performed,  there  should  be  many  questions  to  aid  imagination.  A  student 
who  stands  on  an  inclined  cellar-door  or  plank  shows  the  inclination  of  the 
vertical  body  to  the  plane  on  which  he  stands. 


ANNUAL   MOTIONS  OF   THE  STARS  AND   SUN. 


inclined  to  the  plane  of  the  paper,  its  end  is  at  unequal 
distances  from  the  points  of  the  circumference. 

Now  the  student  can  see  the  circle  of  the  ecliptic  on 
the  heavens,  and  also  the  pole  or  end  of  the  axis,  though 
he  can  not  see  the  plane  or  the  axis.  Thus  he  can  see 
that  different  points  on  the  circle  are  at  unequal  dis- 
tances from  the  pole.  Therefore  the  axis  of  the  celestial 
sphere  is  inclined  to  the  plane  of  the  ecliptic,  and  the 
earth's  axis  is  not  perpendicular  to  the  plane  of  her  orbit. 

Thus  the  earth's  revolution  around  the  sun  resembles 
the  walker's  revolution  round  the  circle  on  the  hill-side. 
Therefore,  we  can  account  for  the  facts  (i)  that  the  sun 
does  not,  like  the  stars,  make  an  annual  revolution  round 
the  earth  ;  (2)  that  the  sun  moves  north  and  south.  When 
the  earth  moves  southeast,  the  sun  moves  not  northwest, 
but  north,  because  east  and  west  are  mere  directions  of 
revolution,  and  the  sun  makes  no  annual  revolution  round 
the  earth. 

It  is  well  for  the  student  to  know  how  these  motions  would 
appear  to  a  person  who  could  stand  off  and  see  them.  To  illus- 
trate them,  a  small,  cheap  globe  may  be  used,  which  is  found  in 
nearly  all  primary  schools.  It  is  on  a  wire  foot.  It  must  be 
placed  on  the  foot  on  a  table,  and  then  moved  slowly  round  the 
table  without  rotation  on  its  axis.  The  student  notes  very  care- 
fully that  every  particle  moves  in  a  horizontal  circle  or  plane, 
parallel  to  the  surface  of  the  table.  The  axis  is  seen  to  be  in- 
clined to  this  plane.  Afterward  the  globe  must  remain  in  one 
place  on  the  table,  and  it  must  be  slowly  revolved  on  its  axis. 
The  student  here  notes  carefully  that  every  revolving  particle 
keeps  at  the  same  distance  from  the  pole,  precisely.  Thus  the 
axis  is  perpendicular  to  the  plane  of  this  motion.  This  may  be 
further  shown  by  taking  up  the  globe  and  holding  it  so  that  the 
axis  is  vertical,  and  then  revolving.  The  student  notes  that  the 
particles  all  move  in  horizontal  circles  when  the  axis  is  vertical. 

Finally,  when  the  student  perfectly  understands  the  posi- 
tion of  the  axis  in  regard  to  the  planes  of  the  motion,  the  two 
motions  may  be  performed  together  on  the  table.  The  globe 
may  be  moved  round  the  table  and  revolved  also,  keeping  the 
line  of  the  axis  all  the  time  extended  in  the  same  direction,  just  as 
the  earth's  axis  always  points  north  and  south. 

41.  One  thing  remains  to  be  accounted  for  in  regard 
to  the  stars.  The  earth's  motion  north  from  June  to 
December  did  not  make  the  stars  appear  to  move  south, 
as  it  made  the  sun.  In  the  experiment  on  the  hill-side 
(39),  when  the  walker  ascends,  not  merely  does  the  head 
at  the  center  seem  to  go  down,  but  surrounding  objects 
seem  to  descend.  But  those  near  at  hand  seem  to  move 
farther  than  those  at  a  distance.  Let  the  student  move 
in  a  straight  line  on  level  ground,  where  he  can  see  near 
and  distant  objects.  All  seem  to  move  in  a  direction 
contrary  to  that  in  which  he  moves ;  but  the  less  remote 
objects  move  much  farther  than  distant  ones.  Very  dis- 
tant hills  do  not  seem  to  move  at  all.  This  makes  ob- 


jects near  at  hand  seem  to  move  over  remote  ones  as  a 
background. 

Thus  the  fact  that  the  sun  moves  north  or  south,  and 
the  stars  do  not,  can  be  accounted  for  by  supposing  that 
the  stars  are  at  an  enormous  distance,  while  the  sun  is 
much  nearer  to  us.  This  also  accounts  for  the  fact  that 
the  stars  do  not  seem  to  change  place  in  regard  to  each 
other,  but  move  as  if  painted  on  the  canvas  of  a  panorama. 

42.  We  have  discussed  the  aspect  of  the  zodiac  on 
the  heavens  at   sunset,   December   22d.      Astronomers 
tell  us  that  the  ecliptic  on  the  heavens  at  sunset  always 
shows  the  direction  in  which  the  earth  has  moved  dur- 
ing the  previous  six  months.     It  is  only  a  little  altered 
at  dark.     If  we  observe  for  any  six  months,  we  find  the 
earth  successively  between  the  sun  and  each  zodiacal 
constellation  seen  on  the  heavens  at  dark  at  the  close  of 
the  time ;  also,  when  the  sun  has  moved  north  for  six 
months,  the  ecliptic  extends  southeast  from  him  at  dark, 
and  when  he  has  moved  south  the  ecliptic  extends  north- 
east.    Thus  the  direction  of  the  earth's  motion  is  always 
contrary  to  the  direction  of  the  sun's  motion,  and  thus 
accounts  for  it. 

It  is  wholly  impossible  for  the  student  to  have  a 
practical  knowledge  of  astronomy  unless  he  knows  the 
ecliptic  in  nature,  and  understands  what  its  various  posi- 
tions mean.  A  littler  further  on  there  will  be  found 
directions  for  its  observation,  and  a  discussion  of  its 
chief  aspects. 

43.  If  the  earth's  center  moves  round  the  sun  on  lines 
extending  from  him  to  the  various  points  of  the  ecliptic, 

FIG.  8. 


.*****  *  **  * 

P •';•:.. 


it  is  clear  that  it  moves  all  in  the  same  plane,  since  a  cir- 
cle  is  a  plane   figure.     Now,  from  Fig.  8  it  is  evident 


20 


ASTRONOMY  BY  OBSERVATION. 


that  the  earth  might  revolve  round  the  sun  in  the  plane 
of  the  ecliptic,  and  yet  move  in  any  one  of  a  great  vari- 
ety of  figures,  differing  very  much  from  a  circle.  But 
in  this  case  the  earth's  distance  from  the  sun  would  vary 
a  good  deal,  and  his  apparent  size  might  be  expected  to 
vary,  like  that  of  objects  on  the  earth  approaching  or  re- 
ceding from  us.  But  we  do  not  notice  any  variation  in 
the  sun's  apparent  size,  as  we  see  him  from  day  to  day ; 
therefore  our  distance  from  the  sun  as  we  revolve  round 
him  can  not  vary  much,  and  the  figure  of  the  earth's 
revolution  must  be  nearly  a  circle. 

44.  There  are  some  facts  which  seem  to  be  an  objec- 
tion to  the  theory  of  the  earth's  annual  revolution  round 
the  sun.  Whenever  we  look  north  we  see  the  pole-star 
in  exactly  the  same  direction  from  us,  just  as  if  the  earth's 
annual  motion  were  an  axial  revolution.  In  order  to  see 
the  force  of  this  objection,  let  the  student  stand  on  one 
spot  and  revolve  axially,  or  turn  round,  and  at  the  same 
time  look  at  the  ceiling.  He  sees  the  same  point  all  the 
time  over  him.  Let  him  next  walk  round  the  room  in  a 
circle,  again  looking  at  the  ceiling.  He  finds  that  differ- 
ent points  are  over  his  head  during  the  revolution. 

The  walker's  axis  is  a  vertical  line  through  his  body. 
As  he  passes  round  a  circle  it  takes  different  positions, 
but  they  are  all  parallel  to  each  other.  Now,  the  earth's 
axis  always  points  north  and  south  ;  therefore  it  is  either 
motionless  or  all  its  positions  are  parallels. 

Let  us  study  parallel  lines.  Let  the  student  stand  at 
the  end  of  any  long  parallel  lines — as  parallel  planks  on 
a  floor,  parallel  rows  of  desks,  parallel  wheel-tracks  on 
a  road,  or  street,  parallel  fences,  parallel  railroad-tracks. 
If  he  sees  these  lines  extend  far  enough,  he  sees  that 
they  appear  to  converge;  and,  doubtless,  if  he  could  see 
them  extending  to  a  very  great  distance,  they  would 
seem  to  converge  to  a  point.  Now,  we  can  account  for 
the  fact  that  the  earth's  axis  always  points  north,  though 
she  moves  in  a  figure  nearly  a  circle,  by  supposing  that 
the  fixed  stars  are  so  very  far  away  that  parallel  lines 
extending  to  that  distance  seem  to  converge  to  a  point. 
Astronomers  have  been  able  to  measure  the  diameter  of 
the  earth's  orbit,  or  path  round  the  sun,  and  have  found 
that  it  is  more  than  184,000,000  miles  in  length.  Thus 
two  parallel  lines  on  which  the  earth's  axis  lies  are  184,- 
000,000  miles  apart ;  but  the  stars  are  so  far  away  that 
the  ends  of  these  lines,  seen  against  them,  appear  to 
converge  to  a  point !  Thus  in  every  direction  we  find 
proofs  of  the  enormous  distance  of  the  fixed  stars.  It 
is  an  essential  part  of  the  theory  of  the  earth's  motion 
around  the  sun. 

45-  If  two  parallel  lines  were  drawn  east  to  the  stars — one 
from  the  point  at  our  feet,  the  other  from  the  point  of  the  earth 
opposite  to  us — the  lines  would  be  8,000  miles  apart.  But  they 


would  appear  to  converge  to  a  point  upon  the  celestial  horizon. 
This  is  the  reason  why  two  people  on  exactly  opposite  points  of 
the  earth  have  the  same  celestial  horizon,  but  different  terres- 
trial horizons. 

46.  The  Earth's  Orbit  an  Ellipse. — The  sun  does 
not  appear  to  vary  in  size  unless  we  measure  it  accu- 
rately. But  measurement  shows  a  small  increase  in  size 
from  June  to  the  end  of  the  year,  and  a  decrease  from 


FIG.  9. 


January  to  June.  From  this  it  is  inferred  that  we  are 
really  nearer  the  sun  in  winter  than  in  summer.  In  a 
good  almanac  the  student  will  find,  about  the  end  of 
December,  the  statement,  "  0  in  perigee  "  ;  and  also,  at 
some  time  in  July,  the  words  "  Q  in  apogee."  They 
mean  that  the  sun  is  at  his  greatest  distance  from  the 
earth  in  July,  and  at  his  smallest  distance  in  January. 
The  same  fact  is  expressed  in  a  different  way  by  saving 
that  "  the  earth  is  in  perihelion  in  December,  in  aphelion 
in  July."  The  difference  in  the  distance  is  estimated  at 
about  3,000,000  miles. 

This  variation  in  the  sun's  apparent  diameter,  with 
some  other  facts,  has  led  to  the  belief  that  the  earth's  orbit 

FIG.  10. 


Circle  and  kllipsts. 

is  an  ellipse.  An  ellipse  is  a  plane  figure  bounded  by  a 
curve  which  has  within  it  two  points,  the  sum  of  whose 
distances  from  any  point  on  the  curve  are  equal.  The 
two  points  are  called  the  foci  of  the  ellipse.  The  great- 
est distance  between  any  two  points  in  a  straight  line  is 
called  the  major  axis  of  the  ellipse ;  the  smallest  dis- 


ANNUAL   MOTIONS   OF   THE  STARS  AND   SUN. 


21 


tance  between  two  points  in  a  straight  line  is  called  the 
minor  axis.  The  eccentricity  of  an  ellipse  will  perhaps  be 
best  understood  by  saying  that  it  is  the  degree  in  which 
the  ellipse  differs  from  a  circle.  An  ellipse  is  drawn  by 
fastening  two  ends  of  a  string  to  paper  firmly  secured  to  a 
board,  and  by  then  placing  the  point  of  a  pencil  against 
the  loop  and  revolving  it.  (See  Fig.  9.)  The  ellipse  in 
which  the  earth  moves  can  hardly  be  distinguished  from  a 
circle.  The  sun  is  at  one  of  the  foci.  (See  Figs.  loand  11.) 

FIG.  ii. 


'j'he  Earth's  Orbit. 

47.  The  Sun  and  the  Ecliptic. — We  know,  from  the 
western  motion  of  the  stars,  that  the  sun  falls  back 
among  them.  Neither  sun  nor  stars  really  move.  They 
only  appear  to  change  position  in  regard  to  each  other 
in  consequence  of  the  earth's  motion.  This  is  called  the 
motion  of  the  sun  in  the  ecliptic.  When  the  stars  seem 
to  change  place  in  regard  to  the  horizon,  it  is  called  the 
motion  of  the  stars.  In  a  year  the  sun  falls  back  com- 
pletely around  the  ecliptic,  and  this  is  called  the  sun's 
revolution  on  the  ecliptic.  In  speaking  of  this  motion, 
we  can  say  that  the  sun  moves  northeast  or  southeast  on 
the  ecliptic,  since  it  is  a  revolution.  But  there  is  no 
appearance  whatever  of  an  annual  revolution  of  the  sun 
round  the  earth,  and  so  we  speak  of  his  motion  on  the 
horizon  as  a  motion  north  or  south. 

Let  us  suppose  that  it  is  sunset,  and  that  a  line  is 
drawn  through  the  earth  and  the  sun  to  the  stars  beyond 
both.  It  would  reach  points  of  the  ecliptic  180°  apart. 
We  should  see  the  sun  at  the  western  end  of  the  line,  and, 
if  there  were  an  observer  at  the  sun,  he  would  see  us  at 
the  eastern  end  of  the  same  line.  The  two  positions,  that 
of  the  sun  as  seen  by  an  observer  at  the  earth,  and  that 
of  the  earth  as  seen  by  an  observer  at  the  sun,  would 
always  be  180°  apart.  The  observer  at  the  sun  would 
seem  to  see  the  earth  move  on  the  ecliptic. 

The  daily  and  annual  motion  of  the  sun  go  on  to- 


gether. He  revolves  daily  with  the  sphere,  and  also 
moves  among  the  stars  on  the  sphere.  This  can  be  illus- 
trated by  supposing  an  ant  to  walk  eastward  round  a 
globe,  while  the  globe  revolves  westward  and  carries  the 
ant  with  it.  It  is  perhaps  better  illustrated  by  the  moon, 
which  revolves  daily  with  the  starry  sphere,  rising  and 
setting,  and  also  moving  among  the  stars  on  the  sphere. 
Her  daily  motion  is  westward,  like  the  sun's,  and  her 
motion  among  the  stars  is  eastward. 

48.  The  sun's  revolution  on  the  ecliptic  measures  our 
year,  and  for  this  reason  that  circle  is  divided  off  in  de- 
grees and  used  for  celestial  measurements.     Circles  par- 
allel to  the  ecliptic  are  called  Celestial  Parallels,  and  cir- 
cles perpendicular  to  it  are  called  Celestial  Meridians. 
Distances  from  the  ecliptic  are  called  Celestial  Latitude. 
Celestial  Longitude  is  distance   measured  east  on  the 
ecliptic  from  the  intersection  of  the  ecliptic  and  equi- 
noctial in  Pisces.     (See  Map  III.)     Right  ascension  is 
reckoned  on  the  equinoctial  east  from  the  same  point. 
On  the  earth  longitude  is  counted  both  east  and  west  to 
1 80°,  but  on  the  heavens  right  ascension  and  longitude 
are  reckoned  east  only,  to  360°.* 

Thus  there  are  three  systems  of  circles  for  celestial 
measurement,  viz.,  the  Horizon  System,  the  Equinoctial 
System,  and  the  Ecliptic  System.  All  have  their  uses. 

49.  The  extreme  southern  point  of   the  ecliptic   in 
Sagittarius  (see  map)  is  reached  by  the  sun  on  Decem- 
ber 22d.     Both  the  time  and  the  place  on  the  heavens 
are  called  the  Winter  Solstice.      The  sun  reaches  the 
extreme  northern  point  of  the  ecliptic  in  Gemini  (see 
map)  on  June  22d.     The  time  and  the  place  are  each 
called  the  Summer  Solstice. 

Since  the  sun  is  due  west  at  sunset  on  March  2ist 
and  September  2ist,  he  must  then  cross  the  equinoctial, 
which  intersects  the  east  and  west  points  of  the  horizon. 
The  sun  crosses  in  Pisces  (see  map)  on  March  2ist,  and 
the  time  and  the  place  are  each  called  the  Vernal  Equi- 
nox. The  sun  crosses  in  Virgo  September  2ist,  and  the 
time  and  the  place  are  each  called  the  Autumnal  Equinox. 

A  great  circle  of  the  Equinoctial  System  passes 
through  the  poles  and  the  equinoctial  points  of  the  eclip- 
tic. It  is  called  the  Equinoctial  Colure.  Another  passes 
through  the  poles  and  the  solstitial  points,  and  is  called 
the  Solstitial  Colure.  Both  are  represented  on  the  maps. 

50.  Observation  of  the  Ecliptic. — It  is  impossible  to 
have  a  practical  knowledge  of  astronomy  without  know- 
ing the  ecliptic  in  nature.     It  is  constantly  unrolled  be- 

*  Since  declination  and  right  ascension,  like  terrestrial  latitude  and  longi- 
tude, measure  distances  north  and  south,  east  and  west,  it  would  have  seemed 
more  appropriate  to  call  them  latitude  and  longitude.  But  names  in  use  can 
not  easily  be  changed,  and  the  very  incongruity  fixes  in  memory  the  use  of 
the  tcims. 


22 


ASTRONOMY  BY  OBSERVATION. 


fore  us.  Maps  and  globes  aid  us  in  preparing  for  in- 
telligent observation,  but  it  would  be  very  absurd  to 
substitute  a  knowledge  of  these  for  intelligent  acquaint- 
ance with  the  circle  in  nature. 

The  ecliptic  has  four  chief  aspects :  At  sunset  on 
June  22d  it  extends  from  northwest  to  southeast,  and  at 
sunset  on  December  22d  it  stretches  from  southwest  to 
northeast.  Of  course,  we  can  not  see  this  at  sunset. 
But,  at  different  times,  these  aspects  are  seen  at  all  hours 
of  the  night.  When  they  are  visible,  Sagittarius  is  always 
on  one  side  of  the  horizon,  Gemini  on  the  other.  Maps 
III  and  IV  do  not  show  the  exact  aspect  for  this  season. 

At  sunset,  March  2ist,  the  ecliptic  extends  nearly  over 
our  heads  across  the  sky,  and  at  sunset,  September  2ist, 
it  makes  a  long  curve  toward  the  southern  point  of  the 
horizon.  Maps  I  and  II  show  exactly  these  aspects  with 
Pisces  and  Virgo  on  the  horizon.  Like  the  two  other 
aspects,  these  are,  at  different  times,  seen  at  all  hours  of 
the  night.  In  observing  them  the  student  must  particu- 
larly note  that,  on  March  2ist,  the  ecliptic  is  very  nearly 
perpendicular  to  the  horizon,  and  that  it  is  very  much  in- 
clined to  the  horizon  on  September  2ist.  When  these  as- 
pects are  seen,  Pisces  and  Virgo  are  always  on  the  horizon. 

About  a  month  before  the  time  of  the  solstices  and 
equinoxes,  there  is  a  favorable  opportunity  for  observing 
the  chief  aspects  at  a  convenient  hour. 

It  is  of  great  aid,  in  giving  definiteness  to  the  stu- 
dent's ideas,  if  he  can  see  the  ecliptic  traced,  in  connec- 
tion with  the  equinoctial  system  of  circles,  on  a  revolving 
globe.  The  small  globe  spoken  of  in  the  Introduction 
will  answer  the  purpose. 

Let  the  student  turn  to  Map  II.  It  shows  that,  dur- 
ing the  six  months  before  September  2ist,  the  earth 
moves  first  southeast,  then  east,  then  northeast.  Ob- 
servation shows  that  at  that  time  the  sun  moves  on  the 
horizon,  first  north,  then  appears  stationary,  then  moves 
south.  Now,  when  the  sun  appears  stationary,  the  earth 
has  certainly  ceased  to  move  north  or  south.  But,  since 
the  stars  continue  to  move  west,  the  earth  is  not  at  rest. 
She  must  move  due  east.  Also,  the  sun  must  continue 
to  fall  back  on  the  ecliptic  if  the  stars  continue  to  move 
west.  He  must  move  due  east.  At  June  22d,  when  the 
sun  is  stationary,  the  earth  is  in  Sagittarius.  The  stu- 
dent can  see  on  Map  II  that  the  course  of  the  ecliptic 
through  Sagittarius  is  nearly  east  and  west.  The  sun 
at  that  time  is  in  Gemini,  and  Map  I  shows  that  the 
ecliptic  also  runs  nearly  east  and  west  through  Gemini. 

NOTE. — The  following  sections  should  be  omitted,  unless  the  student's 
knowledge  of  geometry,  as  taught  in  all  high-schools,  is  thorough.  Where  it 
is,  the  student  is  perfectly  prepared  to  understand  what  follows,  with  due 
study,  and,  unless  the  time  is  too  short,  it  ought  not  to  be  omitted.  If  the 
student  is  to  know  the  ecliptic  in  the  heavens,  he  ought  to  understand  how  it 
is  traced.  The  student  should  point  in  reciting. 


51.  How  the  Sun's  Motion  among  the  Stars  is  meas- 
ured.—The  meridian  of  any  place  is  a  great  circle  passing 
through  its  poles  on  the  heavens,  its  zenith,  its  nadir,  and  the 
north  and   south  points  of  its  horizon.     It   revolves  with  the 
earth  around  its  axis.     We  know  the  exact  time  of  a  revolution, 
the  distance  (360°),  and  therefore  the  rate  of  motion.     There- 
fore in  a  given  time  we  can  tell  exactly  how  far  in  degrees  the 
meridian  has  moved.     By  observation  we  learn  the  interval  of 
time  between  two  successive  appearances  of   the  sun  on  the 
meridian,  and  we  can  tell  the  distance  in  degrees  through  which 
the  meridian   has  passed   in   the   interval.     It   is   always  more 
than  360°,  for  while  the  meridian  revolved  back  to  the  point 
where  the  sun  was,  the  sun  moved  east  on  the  ecliptic,  and  the 
meridian  must  revolve  farther  to  catch  up  with  the  sun.     Sub- 
tracting 360°  from  the  whole  distance,  we  get  the  number  of 
degrees  the  sun  traveled  east  among  the  stars  in  a  solar  day. 

When  the  sun  is  on  the  meridian  above  our  heads,  we  can 
measure  his  distance  in  degrees  from  the  south  point  of  the 
horizon  with  an  instrument  for  measuring  angles.  By  finding 
this  distance  daily,  we  can  detect  his  motion  north  or  south.  By 
adding  to  this  distance  the  distance  of  the  south  point  of  the 
horizon  from  the  celestial  south  pole,  or  the  observer's  latitude, 
we  get  the  sun's  distance  from  the  south  pole. 

52.  The  Sun's  Path  shown  to  be  a  Great  Circle.— 
Let  the  student  examine  the  small   globe  used  in   40  and  note 
the  following  facts:  (i)  The  equator,  ecliptic,  and  meridians, 
each  one,  divide  the  surface  of  the  globe  into  equal  parts,  and 
therefore  they  are  great  circles  of  the  sphere.     (2)  All  great 
circles  bisect  each  other.     (3)  No  small  circle  (as  a  parallel) 
bisects  a  great  circle. 

The  meridian  of  any  place  is  a  great  circle.  The  ecliptic  is 
always  crossing  it  at  two  points:*  one  above,  one  below  the 
horizon.  If  the  sum  of  the  distances  of  these  two  points  from 
the  south  pole  always  equals  180°,  the  sun's  path  bisects  the 
meridian  and  must  be  a  great  circle.  Now,  the  point  below  is 
1 80°  east  of  the  point  above;  and,  therefore,  when  the  sun, 
starting  from  the  point  above,  travels  east  180°,  he  will  be  on 
the  point  now  below.  But  astronomers  have  a  great  many 
times  measured  on  the  meridian  the  sun's  two  distances  from 
the  south  pole  at  points  of  his  path  180°  east  from  each  other, 
and  the  sum  of  these  distances  always  equals  180°.  Therefore 
the  meridian,  revolving  over  the  ecliptic,  is  always  bisected  by 
it,  and  the  ecliptic  must  be  a  great  circle. 

53.  The  Ecliptic  traced  among  the  Stars. — The  me- 
ridian of  any  place  is  always  crossed  by  the  ecliptic  above  and 
below  the  horizon,  and  the  arc  of  the  meridian  between  these 
intersections  equals  180°,  since  the  ecliptic  is  a  great  circle. 
When  the  sun  is  on  the  meridian  at  noon,  we  can  find  the  dis- 
tance of  the  intersection  above,  from  the  south  pole.     (See  51.) 
Subtracting  this  from  180°,  we  get  the  distance  from  the  south 
pole  of  the  intersection  below.     In  half  the  time  of  the  earth's 
rotation,  the  intersection  which  was  below  is  on  the  meridian 
above.    We  know  its  distance  from  the  south  pole,  and  subtract- 

*  The  btudent  who  knows  the  ecliptic  in  nature  has  seen  this. 


INEQUALITY  OF  DAYS  AND  NIGHTS. 


ing  from  this  the  number  of  degrees  measuring  the  terrestrial 
latitude,  we  get  the  distance  of  this  intersection  from  the  south 
point  of  the  horizon.  Measuring  this  ascertained  distance  north 
from  the  south  point,  we  find  the  ecliptic  point,  and,  as  the  stars 

'  are  then   visible,  we  have  found   it  among  the  stars.      Other 

•  points  can  be  found  in  the  same  way. 


CHAPTER    IV. 

INEQUALITY    OF    DAYS    AND    NIGHTS— THE    SEASONS— THE 
PRECESSION    OF   THE    EQUINOXES— TIME. 

54.  Inequality  of  Days  and   Nights. — The  sun  is  at 
his  highest  northern  position  on  June  22d,  and  in  the 
United  States  the  days  are  then  at  their  greatest  length  ; 
the  nights  at  their  shortest.    As  he  moves  south,  the  days 
grow  shorter  and  the  nights  longer ;  but  they  do  not 
become  equal  until  September  2ist,  when  the  sun  at  sun- 
set is  due  west,  just  at  the  point  where  the  equinoctial 
cuts  the  horizon.     He  makes  his  daily  journey  then  on 
the  circle  of  the  equinoctial.     Finally,  on  December 
•22d,  he  has  reached  his   extreme  southern   position, 
23^°  south  of  the  equinoctial,  and  he  makes  his  daily 
revolution  on  the  circle  of  23^°  south.     The  days  are 
then   at   their   shortest,  the   nights  at  their  longest. 
When  he  again  begins  his  annual  journey  north,  the 
clays   increase    in    length;    the  nights  decrease,  until 
March  2ist,  when  the  sun  at  sunset  is  again  due  west 
of   us.     He   has  again  reached    the   equinoctial,   and 
makes  his  daily  journey  on  that  circle.     After  that  the 
days  are  longer  than  the  nights,  and  finally,  on  June 
22d,    he  makes    his   daily  journey  on  the  parallel  of 
23^°  north. 

As  we  travel  north,  the  times  of  the  year  for  long 
and  short  days  and  days  equal  to  nights  do  not  change  ; 
but  the  inequality  of  days  and  nights  increases  until 
we  reach  the  pole,  where  they  become  equal  by  the 
night  and  day  each  becoming  six  months  long. 

If  we  travel  south,  the  inequality  decreases,  until 
at  the  equator  days  and  nights  are  equal.  If  we  con- 
tinue our  journey  south  of  the  equator,  we  shall  find 
the  days  begin  to  differ  in  length  as  they  do  in  the 
northern  hemisphere,  only  the  times  are  reversed  ;  the 
long  days  coming  in  December,  the  long  nights  in  June.* 

55.  In  order  to  understand  the  precise  connection  of 

*  The  inequality  of  days  and  nights  is  here  explained  by  arcs  of  circles  on 
the  celestial  sphere,  because  the  student  sees  this  sphere  and  the  motions  of 
the  sun  and  stars  on  it,  and  thus  has  some  personal  knowledge  of  the  facts  on 
which  the  explanation  is  based.  It  corresponds  to  facts  he  sees  in  nature. 
If  we  draw  a  figure  of  the  earth  with  the  light  and  shadow,  he  can  not  com- 
pare it  with  nature.  He  sees  the  connection  between  the  facts  and  the  con- 
clusion, out  he  takes  the  facts  solely  on  the  authority  of  a  book. 


these  phenomena  with  the  sun's  motions,  the  student 
must  remember  that  the  sun,  in  his  apparent  journey 
north  and  south,  also  makes  at  the  same  time  apparent 
diurnal  revolutions  in  circles  parallel  to  the  equinoctial. 
The  parallels  of  that  great  circle  are  called  diurnal  cir- 
cles, because  sun  and  stars  make  their  diurnal  revolu- 
tions on  those  parallels.  Now,  these  circles  decrease  in 
size,  as  the  student  remembers,  in  proportion  as  they  lie 
farther  from  the  equinoctial  and  nearer  the  poles.  For 
this  reason  we  can  not  learn  anything  about  the  sun's 
various  daily  paths  over  our  sky  above  the  horizon,  by 
comparing  the  lengths  of  the  paths  made  by  stars  across 
the  sky.  If  we  could  compare  the  paths  of  stars  at 
equal  distances  north  and  south  of  the  equator,  the  cir- 
cles would  be  equal  and  the  comparison  hold  good. 
We  must  try  to  compare  the  two  parts  of  the  same  cir- 
cle, one  above  and  one  below  the  horizon. 

56.  We  must  draw  a  diagram  of  the  eastern  hemi- 
sphere of  the  student's  own  sky.     The  circle  drawn  be- 


S.PT. 


low,  Fig.  12,  is  made  from  facts,  every  one  of  which  is 
perfectly  well  known  to  him,  and  he  is  invited  to  con- 
sider all  the  lines  in  the  order  in  which  they  were 
drawn,  and  attest  the  correctness  of  the  representation. 
But  he  must  understand  that  it  is  a  diagram,  not  a  pict- 
ure. The  heavens  are  in  every  place  divided  into  east- 
ern and  western  hemispheres  by  the  meridian  of  the 
place,  a  great  circle  passing  through  zenith  and  nadir, 


ASTRONOMY  BY  OBSERVATION. 


north  and  south  poles,  and  the  north  and  south  points 
of  the  horizon.  The  eastern  hemisphere  of  the  student's 
sky  is  bounded  by  this  great  circle,  the  meridian  ;  there- 
fore we  must  first  draw  a  circle.  And  this  eastern  hem- 
isphere is  divided  into  two  equal  parts,  the  visible  part 
and  the  invisible  part,  by  a  line  called  the  horizon. 
Therefore,  we  draw  a  horizontal  line  across  the  circle. 
The  horizon  intersects  the  meridian  on  the  left  in  the 
north  point ;  on  the  right,  in  the  south  point  (observer 
facing  east).  The  point  of  the  horizon  half-way  between 
the  north  and  south  points  is  called  the  east  point.  We 

FIG.  12. 
ZENITH 


N.PT. 


mark  all  these  points  on  the  diagram.  The  north  pole 
is  on  the  meridian  above  the  north  point.  We  shall  put 
it  at  about  40°  above  the  north  point,  because  in  a  large 
part  of  the  United  States  the  altitude  of  the  pole  does 
not  differ  greatly  from  40°.  The  south  pole  is  on  the 
meridian  at  the  same  distance  below  the  south  point  of 
the  horizon,  so  we  draw  it  there.  The  celestial  sphere 
has  on  it  a  great  circle,  the  equinoctial,  which  is  at  every 
point  on  it  half-way  between  the  poles.  It  of  course 
crosses  the  eastern  hemisphere  of  the  student's  sky,  and 
also  the  meridian.  It  must  cross  the  meridian  at  points 
half-way  between  the  poles.  These  points  are  found, 
and  a  line  is  drawn  between  them.  It  will  be  found  to 
cross  the  horizon  in  the  east  point.  Other  circles  cross 
the  sphere  parallel  to  the  equinoctial.  These  are  the 
diurnal  circles,  and,  as  we  have  the  equinoctial,  it  is  easy 
to  draw  some  circles  parallel  to  it. 


The  student  probably  says  just  here,  "  But  the  part 
of  the  eastern  hemisphere  which  I  see  in  nature  is  con- 
cave, and  this  looks  flat."  That  is  to  say,  the  diagram 
is  not  a  picture.  It  lacks  perspective.  The  author  did 
not  want  a  picture,  because  perspective  depends  on  illu- 
sion. It  would  alter  the  proportion  between  the  parts  of 
the  diurnal  circles  lying  above  and  below  the  horizon,  and 
that  proportion  is  the  very  thing  we  wish  to  know.  The 
diagram  shows  this  more  truly  than  a  picture  would. 

The  sun  is  on  the  meridian  below  the  horizon  at 
midnight.  He  is  on  the  meridian  above  the  horizon  at 
midday.  He  therefore  crosses  the  eastern  hemisphere 
between  midnight  and  midday.  We  will  draw  ar- 
rows, indicating  the  direction  in  which  he  crosses  it. 
We,  of  course,  see  only  half  of  his  daily  revolution,  on 
this  eastern  hemisphere  of  the  heavens.  But  it  is  evi- 
dent that  the  other  half,  on  the  western  hemisphere, 
would  correspond  exactly  with  this.  The  period  be- 
tween sunrise  and  noon  when  the  sun  is  on  the  meri- 
dian above  us,  a  period  which  the  sun  passes  in  the 
eastern  hemisphere,  is  exactly  equal  to  the  period  be- 
tween noon  and  sunset,  which  the  sun  passes  in  the 
western  hemisphere.  Therefore,  it  is  only  necessary 
to  study  one  of  the  hemispheres.  We  will  now  dis- 
cuss the  diagram. 

In  March  and  September  the  sun  is  due  east  from 
us  at  sunrise,  or  at  the  east  point  of  the  horizon.  He 
is  on  the  equinoctial,  and  must  make  his  diurnal  revo- 
lution with  it.  On  the  diagram  the  portion  of  the 
equinoctial  situated  on  the  eastern  hemisphere  is  di- 
vided into  equal  parts  by  the  horizon.  From  mid- 
night to  midday  the  sun  would  evidently  be  just  half 
his  time  above  the  horizon.  This  accounts  for  the 
fact  that  on  March  2ist  and  September  2ist  the  nights 
and  days  are  equal. 

From  March  to  September  the  sun  is  seen,  at  sunrise, 
north  of  the  east  point  of  the  horizon,  and  must  there- 
fore make  his  daily  revolution  on  circles  lying  north  of 
the  equinoctial.  The  diagram  shows  that  less  than  half 
of  each  circle  north  of  the  equinoctial  lies  below  the 
horizon.  The  sun,  in  traveling  on  them,  would  be 
above  the  horizon  more  than  half  the  time.  This  ac- 
counts for  the  fact  that,  from  March  to  September,  the 
days  are  longer  than  the  nights.  The  inequality  be- 
tween the  parts  of  the  circles  lying  above  and  below  the 
horizon  is  greatest  in  those  circles  lying  farthest  north. 
We  should  therefore  expect  the  inequality  between  days 
and  nights  to  be  greatest  when  the  sun  has  reached  his 
extreme  northern  position.  This  is  true.  The  longest 
day  comes  on  June  22d,  and  after  that  the  sun  again 
moves  south. 

From  September  to  March  we  find  the  sun  at  sun- 


INEQUALITY  OF  DAYS  AND  NIGHTS. 


rise  south  of  the  east  point.  The  nights  are  longer 
than  the  days.  The  sun  now  makes  his  diurnal  revolu- 
tion on  circles  lying  south  of  the  equinoctial.  On  look- 
ing at  our  diagram  of  the  eastern  hemisphere,  we  find 
that  more  than  half  of  each  one  of  these  circles  must  lie 
below  the  horizon.  This  accounts  for  the  short  days 
and  long  nights  from  March  to  September.  The  divis- 
ion of  the  circles  on  the  diagram  is  more  unequal  as 
they  are  situated  farther  south.  This  is  evidently  the 
reason  why  the  shortest  day,  December  22d,  comes  when 
the  sun  has  reached  his  extreme  southern  position. 

The  cause  of  the  inequality  of  days  and  nights  is 
evidently — i.  The  sun's  north  and  south  motion.  2.  The 
unequal  parts  into  which  the  diurnal  circles  are  divided 
by  a  horizon  which  does  not  pass  through  the  celestial 
poles. 

57.  Since  all  the  people  living  on  the  terrestrial  paral- 
lel of  40°  north  latitude  have  their  poles  40°  above  the 
horizon,  it  is  clear  that  this  diagram  would  represent  the 
eastern  hemisphere  of  the  sky  for  all  of  them.     Our  an- 
tipodes, or  the  people  on  the  opposite  side  of  the  globe, 
see  the  part  of  the  celestial  sphere  at  any  time  invisible 
to  us.     That  is  represented  by  the  part  of  the  diagram 
below  the  horizon.      The  antipodes  of   all  the  people 
living  in  40°  north   latitude  live  in  40°  south  latitude. 
Therefore,  the  lower  part  of  this  diagram  represents  the 
visible  part  of  this  hemisphere  to  the  people  on  the  ter- 
restrial parallel  of  40°  south  latitude.     They  would  call 
this  the  "  western  hemisphere,"  but  north  and  south  on 
it  are  the  same  directions  to  them  and  to  us.     It  is  clear, 
from  an  examination  of  the  diagram,  why  they  have 
their  long  days  when  the  sun  is  at  the  south,  their  short 
days  when  he  is  at  the  north.     It  is  because  their  day- 
time comes  when  their  antipodes  have  night. 

58.  It  remains  to  account  for  the  fact  that  the  in- 

equality of  days  and 

FIG-  13-  nights   increases    as 

we  move  toward  the 
poles,  and  decreases 
as  we  approach  the 
equator.  The  two 
circles,  Figs.  13  and 
14,  show  the  eastern 
hemisphere  of  the 
heavens  at  Quebec 
and  at  New  Orleans. 
In  drawing  these  dia- 
grams, the  change 
in  the  height  of  the 
poles  made  it  neces- 
sary to  alter  the  position  of  the  equinoctial  (which  is 
half-way  between  the  poles),  and  therefore  of  the  diurnal 
4 


DIURNAL   CIRCLES    AT    NEW    ORLEANS. 


FIG.  14. 


:\ 


circles.     The  student  remembers  that  the  equinoctial  in- 
tersects the  meridian  midway  between  the  north  and 
south  poles.     The  greater  elevation  of  the  pole  at  Que- 
bec makes  the  equinoctial  and  other  diurnal  circles  more 
oblique   to    the   hori- 
zon, which   therefore 
divides      them      into 
more    unequal   parts, 
thus  increasing  the  in- 
equality of  days  and 
nights.     On  the  other 
hand,  at  New  Orleans, ".( 
the  depression  of  the 
pole   (in    comparison 
with    its    positions  in 
latitudes      40°,     45°) 
makes  the  equinoctial 
and     diurnal     circles 
more    nearly  perpen- 
dicular to  the  horizon.     The  two  parts  of  the  diurnal 
circles  are  less  unequal  than  in  Quebec,  and  therefore 
days  and  nights  are  more  nearly  equal  in  New  Orleans. 

59.  The  student  must  take  note  that  in  all  three  of 
the  diagrams  the  equinoctial  is  cut  into  equal  parts  by 
the  horizon.     The  celestial  horizon  is  everywhere  a  great 
circle  of  the  celestial  sphere.     Great  circles  of  the  same 
sphere  always  bisect  each  other.*     Therefore,  the  equi- 
noctial is  in  every  place  divided  by  the  horizon  into 
equal  parts.     For  this  reason,  whenever  the  sun  makes 
his  daily  journey  on  the  equinoctial,  as  in  March  and 
September,  days  and  nights  are  equal  all  over  the  world. 

60.  The  diagram  (Fig.   15)  shows  the  eastern  hemi- 


DIURNAL   CIRCLES   AT   QUEBEC. 


.ES 


IAL 


RIZON 


*  The  student  may  prove  this  with  the  small  terrestrial  globe.  A  great 
circle  of  a  sphere  divides  its  surface  in  half.  The  student  can  not  pass  a 
thread  round  the  globe,  so  as  to  divide  its  surface  in  half,  without  thereby 
bisecting  the  equinoctial,  ecliptic  (so  called),  and  meridian  circles. 


26 


ASTRONOMY  BY  OBSERVATION. 


FIG.  16. 


sphere  of  the  sky  at  the  equator.  Here  the  poles  are  on 
the  horizon,  and  the  equinoctial,  which  intersects  the 
meridian  half-way  between  them,  must  intersect  it  in  the 
zenith  and  nadir.  So  the  equinoctial,  and  therefore  all 
the  diurnal  circles,  are  perpendicular  to  the  horizon, 
which  therefore  divides  them  into  equal  parts.  For  this 
reason,  the  days  and  the  nights  are  always  equal  at  the 
equator. 

61.  At  the  poles,  the  equinoctial  is  no  longer  inter- 
sected by  the  hori- 
zon, but  coincides 
with  it,  and  therefore 
the  diurnal  circles 
are  parallel  to  the 
horizon.  For  this 
reason  we  give  (Fig. 
1 6),  not  an  eastern  or 
western  hemisphere 
for  the  pole,  but  a 
diagram  of  the  whole 
heavens  above  the 
horizon  at  the  north 
pole.  All  the  circles 
5vJ^u~  north  of  the  equinoc- 

tial lie  above  the  ho- 
rizon at  the  north  pole.  Therefore,  when  the  sun  is  north 
of  that  circle,  he  is  always  visible  at  the  north  pole  ;  that 
is,  from  March  to  September.  He  appears  to  wind 
around  the  sky  in  circles  nearly  parallel  to  the  horizon, 
never  getting  more  than  23  y2°  above  the  horizon,  and 
then  winding  back.  When  the  sun  is  south  of  the  equi- 

FIG.  17. 


AND 


62.  It  is  of  interest  to  know  how  the  earth  would 
look  if  we  could  stand  off  from  it  at  different  seasons, 
and  see  the  part  illuminated.  It  is  evident  the  half  next 
the  sun  would  be  enlightened. 

Fig.  17  shows  the  illuminated  half  of  the  earth  at  the 
time  of  the  equinoxes.  The  sun  is  then  vertical  at  the 
equator,  and  the  line  between  light  and  darkness  runs 
through  the  poles. 

Fig.  1 8  represents  the  illuminated  half  of  the  earth  at 
the  time  of  the  winter  solstice.  The  sun  is  vertical  at 

FIG.  18. 


noctial,  he  is  invisible  at  the  north  pole,  since  the  circles 
on  which  he  moves  are  all  below  the  horizon. 

At  the  south  pole  the  conditions  are  similar,  but  the 
times  are  reversed. 


the  Tropic  of  Capricorn,  south  of  the  equator.    The  south 
pole  is  enlightened,  and  the  north  pole  is  in  darkness. 

Fig.  19  exhibits  the  illumination  of  the  earth  at  the 
time  of  the  summer  solstice.     The  sun  is  vertical  at  the 

FIG.  19. 


Tropic  of  Cancer.     The  north  pole  is  illuminated,  and 
the  south  pole  is  in  darkness. 


THE  SEASONS. 


The  terrestrial  circles  called  the  Tropics  of  Cancer 
and  Capricorn,  and  the  Arctic  and  Antarctic  Circles,  all 
indicate  the  sun's  movements.  The  Tropic  of  Cancer  is 
the  extreme  northern  parallel  on  which  the  sun  shines 
vertically ;  the  Tropic  of  Capricorn  the  extreme  south- 
ern parallel.  The  Arctic  and  Antarctic  Circles  mark 
the  boundary  of  sunlight  when  the  sun  is  vertical  over 
the  Tropics. 

63.  The  Annual  Change  of  Temperature.— In  order 
to  understand  thoroughly  some  of  the  effects  of  the  sun's 
motions,  the  sun  should  be  observed  at  noon  on  June 
22d,  and  also  on  December  22d.     This  statement  must 
be  accompanied  by  a  warning  against  looking  at  the  sun 
with  unprotected  eyes.     It  is  in  the  highest  degree  dan- 
gerous.    For  the  present  purpose,  observation  can  be 
very  conveniently  made  through  a  thin  umbrella. 

The  object  of  the  observation  is  to  note  the  sun's 
variation  in  altitude  above  the  south  point  of  the  hori- 
zon, and  its  distance  from  the  zenith. 

In  order  that  the  description  of  the  sun's  motions 
may  excite  real  and  definite  ideas,  the  student  should  go 
out  of  doors  while  studying  this  part  of  the  book,  and 
look  at  the  heavens  as  now  to  be  directed.  Let  him  first 
look  at  the  zenith.  In  the  United  States  the  sun's  posi- 
tion on  June  22d  is  a  little  south  of  the  zenith.  As  the 
student  looks  at  this  point,  he  takes  note  that  a  line 
drawn  from  it  to  himself  would  be,  not  quite  vertical, 
but  a  little  oblique.  Let  him  next  look  at  a  point  about 
30°  from  the  south  point  of  the  horizon ;  that  is,  about 
one  third  the  distance  from  the  horizon  to  the  zenith.  In 
a  large  part  of  the  United  States  the  sun,  on  December 
22d,  is  at  least  60°  from  the  zenith.  As  he  looks  at  this 
point,  let  him  note  that  a  line  drawn  from  it  to  himself  is 
much  more  oblique  than  a  line  drawn  from  the  sun's  po- 
sition of  June  22d. 

64.  Let  us  now  consider  some  other  facts.    The  sun's 
annual  journey  from  north  to  south  coincides  with  a 
change  of  temperature,  called  the  "  change  of  the  sea- 
sons."    There  is  always,  from  January  to  July,  a  general 
increase  of  heat ;  from  July  to  January,  a  general  de- 
crease.    It  is  true  that  summers  differ  in  regard  to  heat, 
while  some  winters  are  colder  than  others.     If,  also,  we 
select  periods  of  two  or  three  weeks,  it  is  not  true  that 
the  day  nearest  to  January  will  always  be  coldest,  the 
day  nearest  to  July  always  warmest.     These  variations 
lead  us  to  think  that  the  sun's  motions  are  not  the  only 
cause  affecting  the  seasons.     But  since  summers  are  al- 
ways warmer  than  winters,  we  are  led  to  think  that  the 
sun's  north  and  south  journey  is  the  cause  of  an  annual 
variation  of  temperature. 

65.  We  gain  some  further  knowledge  of  the  sun's 
effect  on  the  temperature  by  traveling  north  or  south. 


As  we  journey  northward,  the  severity  of  winter  in- 
creases, and  the  heat  of  summer  decreases.  As  we  travel 
southward,  the  contrary  effect  is  perceived,  until  we 
reach  the  equator.  After  crossing  that  circle,  the  effects 
of  moving  north  and  south,  as  seen  north  of  the  equator, 
are  reversed. 

Now,  it  is  evident  that,  as  we  travel  farther  north, 
the  sun  would  seem  to  more  farther  south,  and  thus  the 
obliquity  of  the  sun's  rays  would  increase.  Travelers 
report  to  us  that  this  is  the  case ;  and  thus  again  a  de- 
crease in  heat  coincides  with  increasing  obliquity  of  the 
sun's  rays.  As  we  go  toward  the  equator,  the  obliquity 
decreases,  in  conjunction  with  the  experience  of  warmer 
weather.  After  we  cross  the  equator,  and  travel  toward 
the  south  pole,  the  climate  becomes  colder  and  the  sun's 
rays  more  oblique. 

66.  We  have,  besides  this,  a  daily  experience  of  in- 
creased heat  coinciding  with  a  diminishing  obliquity  of 
the  sun's  rays.     In  the  early  hours  of  the  morning  the 
sun's  rays  are  very  oblique,  and  the  heat  regularly  in- 
creases as  the  sun  becomes  more  nearly  vertical.     All 
these  facts  make  it  impossible  to  doubt  that  the  varying 
obliquity  of  the  sun's  rays,  caused  by  his  north  and  south 
motion,  is  the  direct  means  by  which  the  annual  change 
of  temperature  is  effected. 

67.  If  the  varying  obliquity  of  heat-rays  changes  the 
degree  of  warmth  which  we  feel,  it  ought  to  be  true  of 
rays  from  other  sources  of  heat  than  the  sun.     If  the 
student   will    wet   two   pocket  -  handkerchiefs   of    equal 
thickness,  and  hang  them  up  before  an  open  fire  in  posi- 
tions  equally  favorable   for   receiving  warmth,  except 
that  one  is  hung  parallel  to  the  fire,  the  other  in  an 
oblique  or  slanting  position,  he  will  find  that  the  one 
hung  parallel  will  dry  first. 

68.  It  may  seem  an  objection  to  this  reasoning  that 
the  coldest  weather  comes  about  a  month  after  the  win- 
ter solstice  at  December  22d,  and  the  warmest  about  a 
month  later  than  the  summer  solstice  at  June  22d  ;  and 
also  that  the  warmest  period  of  the  day  is  a  good  deal 
later  than  twelve  o'clock.     But  this  is  only  in  unison 
with  the  fact  that  on  a  cold  day,  when  one  is  perfectly 
warm  and  comfortable,  and  goes  out  into  the  cold,  he 
does  not  at  once  reach  his  coldest  feeling.     Also,  when 
we  take  a  hot  walk  in  summer,  we  do  not  at  once  cool 
off  on  getting  into  the  house.     The  explanation  of  these 
things  is,  we  both  lose  and  gain  heat  gradually.     They 
do  not  affect  the  proof  that  the  varying  obliquity  of  the 
sun's  rays,  caused  by  his  movement  north  or  south,  is 
the  cause  of  the  annual  change  of  temperature  which  we 
call  "  the  Seasons." 

69.  Other  causes  modify  the  variation  of  temperature 
caused  by  the  sun's  obliquity.     Thus  the  climate  of  a 


28 


ASTRONOMY  BY  OBSERVATION. 


place  is  affected  by  altitude  above  the  level  of  the  sea, 
distance  from  the  sea,  and  the  character  of  the  prevail- 
ing winds.  In  going  north  or  south,  we  may  find  some 
of  these  causes  more  effectual  in  some  places  than  the 
sun's  increasing  or  decreasing  obliquity. 

70.  The  heat  of  summer  is  evidently  much  affected 
also  by  the  length  of  days,  causing  heat  to  accumulate, 
since  we  gain  and  lose  it  gradually.    As  we  travel  north, 
the  days  and  nights  grow  more  unequal.     For  this  reason 
it  is  often  found  that  Quebec  or  Montreal  will  report,  on 
one  or  two  days  in  July,  the  mercury  at  a  higher  degree 
in  the  thermometer  than  Charleston  or  Savannah.      But 
the  general,  or  average  heat  of  the  summer  is  much 
greater  at  Charleston   and   Savannah,  and   the   season 
longer. 

71.  The  student  remembers  that  the  apparent  diam- 
eter of   the    sun  is  greater   in   our  winter  than  in  our 
summer,  indicating  that  the  sun  is  in  perigee,  or  nearest 
to  the  earth,  in  winter,  and  in  apogee  in  summer.     Our 
experience  shows  that  our  distance  from  the  sun  does 
not  vary  enough  to  counteract  the  effect  of  variation  in 
the  obliquity  of  the  sun's  rays.     But  in  the  southern 
hemisphere  the  earth  is  in  perihelion  in  summer,  in  aphe- 
lion in  winter.     If,  now,  the  variation  in  our  distance 
from  the  sun  affects  the  earth  at  all,  it  might  be  expected 
to  make  the  extremes  of  temperature  in  both  winter  and 
summer  a  little  greater  in  the   southern  than  in  the 
northern  hemisphere.     It  takes  long,  careful,  and  wide- 
ly extended  observations  to  settle  this  question ;  for  in 
going   from  one  locality  to  another,  altitude,  distance 
from    the    sea,    and    prevailing    winds,   all   affect   the 
climate   of  a  place ;    but  it  is  considered  certain   that 
the   winter  and   summer   of    the   southern  hemisphere 
are  both  more  extreme  than  the  seasons  of  the  north- 


ern. 


72.  Inequality  in  the  Sun's  Motion. — The  earth  does 
not  move  in  her  orbit  through  equal  distances  in  equal 
times,  and,  of  course,  the  sun  moves  with  unequal  speed 
in  the  ecliptic.  This  is  learned  in  the  following  way : 
When  the  sun  is  due  west  at  sunset,  he  is  crossing  the 
equinoctial.  Now,  astronomers,  by  watching  the  sun  ex- 
actly at  the  time  he  crosses  the  meridian  at  midday,  and 
then  taking  his  angular  distance  from  the  south  point 
of  the  horizon,  can  tell  with  minute  accuracy  when  he 
crosses  the  equinoctial,  since  they  know  exactly  how  far 
it  is  from  the  south  point  of  the  horizon.  They  find  the 
sun  on  the  equinoctial  twice  a  year,  in  March  and  Sep- 
tember. As  the  points  where  the  equinoctial  intersects 
the  ecliptic  are  180°  apart  on  each  circle,  it  is  evident 
that,  in  the  intervals  between  the  sun's  two  passages 
across  the  equinoctial,  he  has  traveled  through  equal 
spaces.  But  the  student  may  count  the  days  from  March 


2  ist  to  September  2ist,*  and  then  again  from  September 
2  ist  to  March  2ist,  and  he  will  find  the  first  interval  a 
few  days  the  longest.  From  March  to  September,  the 
sun  travels  north  of  the  equinoctial,  therefore  he  takes 
a  longer  time  to  pass  over  the  180°  lying  north  than 
the  180°  south  of  it.  The  times  being  unequal,  and  the 
distances  equal,  the  rate  of  motion  varies. 

73.  By  looking  in  the  almanac,  the  student  will  find 
the  words  "  O  in  perigee,"  "  O  in  apogee,"  in  December 
and  July.     These  words  indicate  the  times  of  the  earth's 
smallest  and  greatest  distances  from  the  sun.     The  time 
when  the  earth  moves  fastest  thus  coincides  with  her 
greatest  approach  to  the  sun.     Astronomers  attribute 
the  earth's  varying  speed  to  a  variation  in  the  attraction 
of  the  sun  produced  by  his  varying  distance  from  her. 

74.  Slow  Changes  of  Motion. — The  earth's  motions 
undergo  some  changes  so  slow  that  the  observations  of 
many  generations  of  astronomers  are  needed  to  detect 
and  know  them  accurately.    Two  will  be  mentioned  here. 

75.  The  earth  is  in  perihelion,  or  at  the  point  of  her 
orbit  nearest  the  sun,  very  nearly  at  the  time  of  the  win- 
ter solstice.f     But  the  times  of  perihelion  and  aphelion 
come  a  few  minutes  earlier  every  year.     In  the  course 
of  several   thousand  years,  the  earth's  perihelion  will 
have  revolved  round  the  year  until  it  comes  at  the  time 
of  the  autumnal  equinox,  when  the  climates  of  the  north- 
ern and  southern  hemispheres  will  become  equalized. 
After  a  still  longer  period  the  time  of  perihelion  will 
come  in  the  summer  of  the  northern  hemisphere.     That 
difference  in  the  climates  of  the  two  hemispheres  which 
is  caused  by  the  variation  in  the  earth's  distance  from 
the   sun  will   then   be  reversed  (see  71).      Finally,  the 
perihelion-point  will  revolve  back  to  its  present  position, 
and  the  present  climates  will  be  restored.     If  a  line  be 
drawn  between  the  points  of  perihelion  and  aphelion, 
the  effect  of  this  change  is  to  make  it  revolve.     The 
change  is  called  a  revolution  of  the  line  of  apsides. 

76.  Precession    of   the     Equinoxes.  —  Astronomers, 
watching  the  sun's  motions  with  a  patience,  carefulness, 
and  persistence  of  which  students  can  form  little  idea, 
discovered  that  the  sun  does  not  cross  the  equinoctial 
at  the  same  points  of  the  ecliptic,  but  a  little  farther 
west  every  year,  and,  of  course,  a  very  little  sooner. 
The  difference  is  only  50",  but  in  about  twenty-five  thou- 
sand years  it  will  make  the  equinoctial  points  revolve 
round  the  ecliptic.     They  also  found  that  certain  stars 

*  Owing  to  our  way  of  measuring  time,  we  can  not  get  the  difference  in 
time  quite  correctly  in  this  way.  Still,  the  difference  is  on  the  right  side,  and 
the  student  will  realize  better  what  he  is  studying  if  he  actually  counts  the 
days. 

f  The  student  must  find  in  the  almanac  the  words  "  ©  in  perigee,"  "  O  in 
apogee." 


PRECESSION  OF  EQUINOXES. 


29 


round  the  north  pole  seemed  to  be  moving  in  circles  of 
increasing  size,  and  certain  others  in  circles  of  decreas- 
ing size.  Since  the  size  of  the  circles  depends  on  the 
distances  from  the  poles,  this  indicates  some  change  in 
the  points  of  the  heavens  between  which  the  earth's  axis 
extends.  Since  the  pole  is  90°  from  every  point  of  the 
equinoctial,  it  is  evident  a  change  in  the  position  of  that 
circle  must  change  the  places  of  the  poles,  or  centers  of 
apparent  motion  in  the  heavens. 

77.  While  the  equinoctial  points  move  west  round  the  eclip- 
tic, the  pole  of  the  heavens  describes  a  circle  with  a  radius  of 
23%°  around  the  pole  of  the  ecliptic.     The  student  may  easily 
understand  how  this  is  by  a  little  study  of  one  of  the  small 
globes  now  very  generally  used,  and  having  a  circle,  called  the 
ecliptic,  marked  on  it.     By  holding  the  globe  so  that  the  ecliptic 
circle  is  perfectly  horizontal,  the  pole  of  the  ecliptic  is  at  the 
top,  and  it  may  be  marked  with  a  little  wax,  so  that  it  can  be 
identified.     Then  the  axis  is  made  perpendicular,  and  it  will  be 
seen  that  the  pole  of  the  ecliptic  revolves  around  the  north  pole 
of  the  globe  when  the  globe  is  revolved.     Let  us  suppose  now 
that  the  ecliptic  moved  round  on  the  sphere,  not  with  it.     It  is 
clear  that  it  would  have  precisely  the  positions  in  space  which 
it  had  when  it  revolved  with  the  sphere,  and,  of  course,  its  pole, 
which  is  90°  from  the  ecliptic  itself,  would  have  precisely  the 
positions  it  did  when  ecliptic  and  sphere  revolved  together — 
that  is,  the  positions  in  space.    And  it  is  clear  that  as  the  ecliptic 
would  slide  round  the  sphere,  the  points  where  it  intersects  the 
equinoctial  moving  round  the  equinoctial,  so  the  pole  would 
slide  round  the  north  pole,  making  a  circle  on  the  sphere. 

Now,  in  explaining  this,  we  have  made  the  ecliptic  move  on 
the  equinoctial.  This  was  done  only  because  the  globe  revolves 
round  the  poles  of  the  equator,  and  could  not  be  made  to  re- 
volve round  the  poles  of  the  ecliptic.  But  in  the  heavens,  the 
student  must  remember  it  is  the  poles  of  the  ecliptic  which  are 
at  rest,  and  the  poles  of  the  equinoctial,  in  this  very  slow  change 
of  which  we  speak,  revolve  round  the  poles  of  the  ecliptic  on 
the  celestial  sphere.  Since  the  north  pole  of  the  ecliptic  is  dis- 
tant 231/.0  from  the  north  pole  of  the  equinoctial,  it  is  clear  that 
the  circle  described  by  the  pole  of  the  latter  would  have  a 
radius  of  23'/s°. 

78.  From  these  facts  it  is  clear  that  the  centers  of  mo- 
tion on  the  celestial  sphere,  or  the  poles  of  the  heavens, 
are  all  the  time  changing  place,  but  so  slowly  that  the 
north  pole  will  not  complete  a  revolution  in  less  than 
twenty-five  thousand  years.     The  map  appended  (Fig. 
20)  shows  the  position  of  the  circle  in  which  the  pole  is 
moving.     The  cipher  shows  the  place  of  the  pole  at  the 
beginning  of  the  Christian  era.     The  figures  to  the  right 
show  the  position  of  the  pole  in  years  B.  c.     Those  to 
the  left  show  the  position  in  years  A.  D.     It  can  be  seen 
that  Polaris  will  be  at  the  center  of  motion  in  the  year 
A.  D.  2000.     In  2850  B.  c.,  a  star  in  Draco  was  the  pole- 
star.     This  is  supposed  to  be  about  the  time  when  the 


Great  Pyramid  was  built.    The  star  Vega  in  Lyra,  of  the 
first  magnitude,  must  at  one  time  have  been  very  near 


FIG.  20. 


^  q    «B  -. 

'ftf  *  \GREATi 

-V  ^5  \  ««"  i 

<X 


^ 


=  -'  '%    *^ 

I0«^ 

REvoLO-r>o" 


the  pole.  Since  the  radius  of  this  circle  is  23^°,  it  is 
clear  that  the  present  pole-star  will  one  day  be  47°  from 
the  pole. 

79.  When  the  ecliptic  was  first  laid  off  in  degrees, 
the  circle  was  divided  also  into  twelve  parts  called  signs, 
and   containing  30°  each.     The  signs  were  named  for 
the  constellations  they  were  then  in,  viz.,  Aries,  Taurus, 
Gemini,  Cancer,  Leo,  Virgo,  Libra,  Scorpio,  Sagittarius, 
Capricornus,  Aquarius,  and  Pisces.     Since  then,  by  the 
precession  of  the  equinoxes,  the  signs  have  moved  out  of 
the  constellations  from  which  they  took  their  names,  but 
they  have  not  changed  names.     Thus,  the  intersection  of 
the  two  great  circles  is  called  the  "  first  point  of  Aries," 
though  it  is  in  the  constellation  Pisces.     The  same  names 
are  used  for  both  signs  and  constellations ;  and  when  we 
hear  them,  it  is  necessary  to  be  careful  in  understanding 
which  is  meant,  since  they  no  longer  coincide.     When 
the  terms  are  used  in  almanacs  in  regard  to  the  sun's 
motions,  the  signs,  not  the  constellations,  are  meant. 

80.  When  the  Tropics  of  Cancer  and  Capricorn  were 
named,  the  winter  solstice,  at  which  time  the  sun  is  ver- 
tical at  the  Tropic  of  Capricorn,  took  place  when  the 
sun  was  in  Capricornus.     Also,  the  sun  was  then  in  Can- 
cer when  he  was  vertical  at  the  Tropic  of  Cancer ;  that 
is,  at  the  summer  solstice.     But  the  precession  of  the 
equinoxes  has  changed  that.     On  the  22d  of  June  the 
sun  is  now  in  Gemini,  not  Cancer ;  and  on  December 
22d  he  is  now  in  Sagittarius,  not  Capricornus.     But  the 
names  are  retained  for  the  two  circles. 


ASTRONOMY  BY  OBSERVATION. 


Since  Celestial  Longitude  and  Right  Ascension  are 
reckoned  from  a  movable  point,  the  Celestial  Longitude 
and  Right  Ascension  of  all  stars  are  slowly  changing. 
Since  the  pole  changes,  Declination  also  changes. 

81.  Time. — Even  savages  need  to  measure  time  in 
some  rough  way,  and  as  the  concerns  of  civilized  life 
become  complicated,  men  want  measures  of  time  which 
shall  be  conspicuous  in  their  application,  and  also  ac- 
curately adapted  to  men  in  all  places  and  all  centuries. 
The  conspicuous  measures  of  time,  corresponding  also 
to  the  practical  affairs  of  life,  are  a  revolution  of  the 
seasons,  and  an  apparent  diurnal  revolution  of  the  sun 
round  the  earth.     Civilized  nations  early  began  to  use 
these,  and  a  desire   to  know   them   and   to  make  our 
divisions  of  time  accurate,  encouraged  the  study  of  as- 
tronomy. 

A  revolution  of  the  seasons  is  not  exactly  equal  to  a 
revolution  of  the  earth  in  her  orbit  round  the  sun.  If 
the  earth  were  between  the  sun  and  that  point  of  the 
ecliptic  which  is  now  crossed  by  the  equinoctial,  she 
would  have  made  a  revolution  round  the  sun  when  she 
again  came  between  the  sun  and  that  point  of  the  ecliptic. 
But  since  the  equinoctial  does  not  continue  to  cross  the 
ecliptic  at  the  same  point,  but  crosses  earlier,  it  is  plain 
that  the  earth  will  get  back  to  the  new  crossing  before 
she  comes  in  line  with  the  stars,  and  the  point  where  she 
formerly  crossed.  This  results  from  what  is  called  the 
precession  of  the  equinoxes.  The  interval  between  the 
two  times  when  the  earth  passes  between  the  sun  and 
any  point  of  the  starry  heavens  is  called  a  Sidereal  Year. 
The  interval  between  two  passages  over  the  same  equi- 
nox is  called  a  Tropical  Year,  and  it  contains  exactly 
one  revolution  of  the  seasons.  A  sidereal  year  contains 
365  days,  6  hours,  9  minutes,  9  seconds.  A  tropical  year 
contains  365  days,  5  hours,  48  minutes,  46  seconds.  The 
difference  is  very  small,  and  would  for  a  long  time  occa- 
sion no  inconvenience  if  the  year  of  our  calendar  cor- 
responded with  the  sidereal  year,  but  in  time  it  would 
make  the  seasons  begin  on  very  different  days  and  hours 
from  those  which  now  mark  their  beginning.  Thus  it 
is  necessary  to  take  the  tropical  year  for  the  basis  of  our 
calendar. 

82.  The  student  recalls  the  fact  that  a  solar  day  is 
longer  than  a  sidereal  day.     We  have,  in  this  chapter, 
discussed    the  inequality  of  days  to  nights,  using  the 
word  day  to  refer  to  the  period  of  light  in  distinction 
from  the  period  of  darkness.     We  shall  now  use  it  to 
refer  to  the  apparent  diurnal  revolution  of  the  sun  round 
the  earth.     It  is  the  interval  between  two  passages  by 
the  sun  over  the  meridian  below  the  horizon. 

83.  The  meridian  of  any  place  is  a  circle  belonging  to 
the  horizon  system  of  circles.     It  revolves  with  the  earth 


on  her  axis,  passing  through  every  point  of  the  ecliptic, 
yet  all  the  time  passing  through  our  zenith  and  nadir. 
Let  us  suppose  the  sun  to  be  on  the  half  of  the  meridian 
which  is  below  the  horizon.  The  meridian,  of  course, 
passes  through  a  certain  point  of  the  ecliptic,  since  the 
sun  is  always  on  the  ecliptic.  The  meridian  makes  a 
revolution  along  with  the  earth,  and  reaches  the  point 
of  the  ecliptic  where  it  left  the  sun.  The  sun,  which  all 
the  time  moves  east  on  the  ecliptic,  is  no  longer  at  that 
point,  but  further  ahead.  Now,  the  student  remembers 
that  the  earth  in  her  orbit  does  not  move  through  equal 
spaces  in  equal  times.  Of  course,  the  sun's  motion  in 
the  ecliptic,  which  is  due  to  the  earth's  motion  in  her 
orbit,  is  of  unequal  speed.  Sometimes  he  will  be  farther 
ahead  than  at  other  times,  and  as  the  meridian,  moving 
by  the  earth's  diurnal  motion  (which  does  carry  her 
through  equal  spaces  in  equal  times),  sometimes  meets 
the  sun  sooner,  sometimes  later,  the  solar  days  vary  in 
length.  But  they  vary  from  another  cause.  The  merid- 
ian moves  due  east,  but  the  sun  in  the  ecliptic  does  not 
move  due  east.  His  motion  is  compounded,  the  student 
remembers,  of  motions  east  and  north  or  south,  and 
sometimes  he  moves  more  directly  east,  sometimes  his 
motion  has  more  of  a  northern  and  southern  direction, 
even  though  he  moves  through  the  same  distance  in 
both  cases.  When  his  motion  is  nearly  east,  as  at  the 
solstices,  it  takes  the  meridian  longer  to  catch  up  than 
when  his  motion  is  nearly  north  and  south,  as  at  the 
equinoxes.  For  this  reason  the  solar  day  (which  is  a 
period  of  both  light  and  darkness)  varies  in  length,  being 
longest  at  the  solstices,  when  it  takes  longest  to  catch  up 
with  the  sun.* 

84.  On  account  of  this  variation  in  the  length  of  the 
solar  days,  the  day  adopted  for  our  calendar  is  the  mean,  or 
average,  of  all  the  solar  days  in  a  year.     An  hour  is  the 
twenty-fourth  part  of  a  day. 

85.  For  this  reason  clock-time  and  sun-time  do  not 
always  agree — or,  in  other  words,  mean  solar  time  and 
apparent  solar  time  do  not  always  agree.     But  the  alma- 
nac always  tells  us  how  they  differ.     (It  is  impossible  to 
study  this  book  well  without  actually  using  an  almanac.) 
If  we  turn  to  February  loth,  we  find  from  the  almanac 
the  time  of  sunrise  and  sunset  given  according  to  our 
clock-time.     But  if  we  subtract  from  twelve,  the  hours 
and  minutes  given  for  sunrise,  in  order  to  find  out  how 
long  it  is  before  twelve  o'clock,  we  should  expect,  if  our 


*  There  are  two  things  the  student  must  not  get  confused.  Days  (periods 
of  light)  are  longest  and  shortest  at  the  solstices,  because  their  length  depends 
on  the  sun's  north  and  south  position.  Days  (periods  of  time  between  the 
sun's  passage  of  the  meridian  below  the  horizon)  are  longest  at  the  solstices, 
because  their  length  depends  on  the  sun's  eastern  motion,  which  is  greatest 
at  the  solstices. 


THE  MOON  AND  HER  MOTIONS,   AND  HOW   TO   OBSERVE   THEM. 


twelve  o'clock  corresponded  with  the  sun's  passage  over 
the  meridian  above  us,  to  find  it  equal  to  the  hours  and 
minutes  given  for  sunset.  The  sun  must  reach  the  me- 
ridian half-way  between  sunrise  and  sunset.  Thus  we 
should  find  that,  on  February  loth,  a  good  watch,  set  by 
the  sun  at  sunrise,  would  not  reach  twelve  at  the  time  the 
sun  crossed  the  meridian.  On  February  loth  the  student 
will  find  that  the  interval  between  sunrise  and  the  noon 
of  the  clock,  is  full  thirty  minutes  longer  than  the  inter- 
val between  the  same  noon  and  sunset.  In  a  column  at 
the  side  of  the  record  of  sunrise  and  sunset,  and  under 
the  heading  "Sun  Slow,"  will  be  found  the  figure  15. 
If  this  be  subtracted  from  the  longer  period  and  added 
to  the  smaller,  either  will  then  give  the  true  time  of  the 
sun's  passage  across  the  meridian,  which  is  found  to  be 
fifteen  minutes  later  than  twelve  o'clock.  Because  the 
sun  comes  to  the  meridian  later  than  the  twelve  of  clock- 
time,  he  is  said  to  be  "slow."  The  column  of  figures  is 
called  the  "  equation  of  time,"  because  adding  and  sub- 
tracting it  makes  the  morning  and  afternoon  periods,  as 
reported  by  the  almanac,  equal. 

By  studying  the  sunrise  and  sunset  figures  of  Novem- 
ber 2d,  we  perceive  another  discrepancy.  Here  the 
almanac  tells  us  the  sun  is  "  fast"  ;  that  is,  he  crosses  the 
meridian  before  the  twelve  of  our  clocks  and  watches 
set  by  the  almanac  figures  for  sunrise.  The  figures 
under  the  sun  column  "  fast"  must  be  added  to  the  after- 
noon figures  and  subtracted  from  the  morning  figures. 
The  difference  between  sun  and  clock  time  is  greatest 
February  loth,  November  2d,  May  Hth,  and  July  25th. 
Four  times  in  the  year  the  almanac  reports  no  difference 
between  the  sun's  actual  passage  across  the  meridian 
and  the  twelve  of  the  clocks.  These  days  are  April  1 5th, 
June  I4th,  August  3ist,  and  December  24th.  The  solar 
and  the  calendar  day  are  then  equal. 

86.  After  we   accept   the   mean   solar  day  and    the 
tropical  year  for  the  basis  of  our  calendar,  another  diffi- 
culty arises.     For  practical  purposes  it  is  necessary  to 
have  our  year  exactly  divisible  by  our  day.     Now,  in 
the  tropical  year,  there  are  365  solar  days,  5  hours,  48 
minutes,  46  seconds.     The   Romans  at  first  made   365 
days  a  year,  but  the  repeated  neglect  of  hours,  minutes, 
and  seconds  every  year,  made  an  increasing  error,  so 
that  in  the  time  of  Julius  Caesar  the  interval  of  a  year 
no  longer  corresponded  with  a  revolution  of  the  seasons. 
Caesar  added  a  day  to  every  fourth  year  (or  six  hours 
for  each  year).      This  year  of  366  days,  called  by  us 
"leap-year,"  is  also  called  a  "Julian  year."     The  calen- 
dar, thus  reformed,  was  used  by  Christian  nations  for 
about  sixteen  hundred  years.     This  calendar  is  called 
"  Old  Style,"  or  simply  "  O.  S." 

87.  But  since  the  tropical  year  is  shorter  than  365 


days,  6  hours,  this  plan  added  too  much,  and  after  a 
while,  March  2ist  got  ten  days  ahead  of  the  sun's  pass- 
age of  the  equinoctial.  Then  Pope  Gregory  introduced, 
A.  D.  1582,  what  is  called  from  him  the  "  Gregorian  Cal- 
endar." This  provides  that  "  every  year  divisible  by  4. 
shall  contain  j66  days,  except  the  centuries,  which  shall  not 
be  leap  years  unless  they  are  divisible  by  4.  after  striking  off 
the  two  right-hand  figures''  Thus,  2000, 2400, 2800,  are  leap- 
years,  but  1900,  2100,  2200,  2300,  2500,  and  2700,  are  not 
leap-years,  though  divisible  by  four.  Even  by  this  rule, 
the  years  are  not  exactly  divisible  by  days,  but  the  error 
will  not  amount  to  a  day  in  about  three  thousand  years. 

88.  But  it  was  necessary,  in  1582,  to  get  rid  of  the 
result  of  past  errors,  as  well  as  to  provide  against  future 
errors.     The  ten  days  which  the  2ist  of  March  had  got 
ahead  of  the  sun's  passage  of  the  equinoctial  were  got 
rid  of  by  calling  the  5th  of  October  the  I5th. 

89.  The  new  mode  of  reckoning  time  is  called  "  New 
Style,"  or  "  N.  S."     It  was  adopted  at  once  by  nearly  all 
European  nations.     But  the  English  people  did  not  adopt 
New  Style  until  1752,  when  the  error  amounted  to  eleven 
days,  which  were  left  out  of  the  year  by  calling  the  3d 
of  September  the   I4th.     New  Style  has  not  yet  been 
adopted  in  Russia. 

90.  Thus  the  year  and  day  of  our  calendar  can  not  in 
strictness  be  said  to  coincide  with  the  tropical  year  and 
solar  day.     The  interval  between  the  sun's  two  succes- 
sive passages  over  the  same  equinox,  and  the  interval 
between   the   sun's   two   successive   passages   over   the 
meridian  below  the  horizon,  are  simply  the  basis  of  our 
year  and  day.     Our  years  and  days  differ  from  the  solar 
year  and  day,  but  some  days  and  years  are  longer,  some 
shorter.     In  the  case  of  the  days,  the  differences  coun- 
terbalance one  another  exactly  ;  in  the  case  of  the  years 
very  nearly.     The  coincidence  is  near  enough  for  practi- 
cal purposes. 

NOTE. — In  the   almanac   the  signs  of  the   zodiac  have  the  following 
symbols  : 

<T>     Aries  <S>     Cancer  ^=     Libra  V3     Capricornus 

«     Taurus  SI     Leo  HI     Scorpio  £?    Aquarius 

n     Gemini  m     Virgo            t      Sagittarius  X     Pisces 


CHAPTER    V. 

THE    MOON   AND    HER    MOTIONS,   AND    HOW    TO    OBSERVE 

THEM. 

91.  No  other  heavenly  body  offers  so  convenient  an 
opportunity  for  full  observation  as  the  moon.  She  comes 
often,  and  goes  through  the  round  of  her  motions  in  a 
reasonable  time.  It  is  not  only  exceedingly  interesting 


ASTRONOMY  BY  OBSERVATION. 


work  to  watch  her  through  a  full  revolution,  it  is  ex- 
cellent preparatory  training  for  the  observation  study  of 
the  planets.  The  positions  of  the  moon  in  relation  to  the 
sun  and  earth  are  similar  to  certain  important  positions 
of  the  planets  in  regard  to  sun  and  earth. 

In  order  to  observe  the  moon  well,  it  is  necessary  to 
know  at  least  those  constellations  of  the  zodiac  which 
are  in  the  evening  sky.  They  can  be  learned  in  a  few 
evenings  by  drawing  them  a  good  deal.* 

It  is  best  to  begin  observation  as  soon  after  the  date 
given  in  the  almanac  for  new  moon  as  the  observer  can 
get  a  sight  of  her.  It  is  very  difficult  to  see  the  new 
moon  before  she  is  two  days  old. 

There  are  three  lines  of  observation  which  must  here 
be  treated  separately,  in  order  to  avoid  the  confusion 
which  results  from  describing  several  things  at  once. 
But  the  student  must  carry  out  all  three  together  when 
he  can  study  nature  for  himself. 

/.   The  Moons  Motions. 

92.  The  Diurnal  Revolution. — At  two  or  three  days 
from  the  time  of  new  moon,  the  moon  will  appear  as  a 
slender  crescent,  seen  soon  after  sunset  above  the  western 
horizon,  and  not  far  from  the  sunset-point.  If,  after  finding 
her,  the  student  will  return  in  ten  minutes  to  look  again, 
he  will  see  that  she  is  moving  toward  the  western  hori- 
zon, and  will  disappear  behind  it  soon  after  sunset.     As 
all  the  heavenly  bodies  which  the  student  has  observed 
exhibit  similar  motions,  he  will  recognize  hers  as  appar- 
ent, and  due  to  the  earth's  rotation  on  her  axis.     If  so, 
the  moon  must  cross  the  sky  below  the  horizon  during 
the  twelve  hours  after  her  disappearance,  pass  above 
the  eastern  horizon  a  little  after  sunrise  (invisible  because 
of   the   sun's  blinding  light),  and,  crossing  the  heavens 
above  the  horizon  during  the  day,  reappear  in  the  west 
when  the  sun's  light  is  withdrawn.     This  is  the  moon's 
apparent  diurnal  revolution,  and  it  is  the  cause  of  her 
rising  in  the  east  and  setting  in  the  west.     As  it  is  desir- 
able to  avoid  any  confusion  of  this  motion  with  others, 
it  will  be  best  to  make  observations  at  the  same  hour 
every  evening. 

93.  The  Moon's  Real  Motion. — On  the  second  night 
of  observation,  the  student  would  find  the  moon  again 
in  the  west,  but  she  would  be  a  little  higher  above  the 
horizon,  that  is,  a  little  farther  east,  than  before.     It  is 
clear  that  the  moon,  or  the  horizon  and  earth,  must  have 
moved.     If  this  motion  is  apparent,  it  must  be  caused 
by  the  motion  of  the  observer  and  earth ;  and  the  earth 
must  move  west  to  cause  it.     But  we  know  that  the 
earth  moves  east.     For  this  reason  we  conclude  that  this 

*  See  Introduction,  "  How  to  Learn  and  Teach  the  Constellations." 


is  a  real,  or,  as  astronomers  call  it,  a  "  proper  "  motion  of 
the  moon.  It  causes  the  moon  to  change  place  among 
the  stars  every  evening,  while  the  diurnal  revolution 
makes  the  moon  move  with  the  stars.  It  should  be  part 
of  the  student's  observation-work  to  note  the  moon's 
place  among  the  stars  every  evening,  and  thus  detect 
her  real  motion. 

94.  The  Moon's  Real  Revolution.     Opposition  and 
Conjunction. — Since  the  moon  was  found  moving  east, 
the  student  will,  upon  reflection,  conclude  that  she  must 
have  come  into  the  evening  sky  from  below  the  western 
horizon,  and,  in  doing  so,  must  have  passed  the  sun.    She 
must  have  crossed  a  meridian  circle  running  north  and 
south  through  the  sun's  position.     When  she  does  this, 
she  is  said  to  be  in  conjunction  with  the  sun.*     She 
does  not  always  cross  this  line  exactly  at  sunset ;  but  as 
she  moves  very  slowly,  she  never  gets  far  east  from  the 
sun  at  the  very  next  sunset  after  it.     So,  after  conjunc- 
tion, we  always  see  her  near  the  western  horizon  at  sun- 
set.    The  moon  becomes  new  after  conjunction. 

If  the  student  continues  to  watch  the  moon's  real 
motion  at  dark,  he  will  find  her  every  evening  farther 
east  among  the  stars ;  and  in  fifteen  days  she  will  have 
crossed  the  evening  sky,  and  will  be  found  at  sunset  on 
or  near  the  eastern  horizon.  She  will  have  made  a  half- 
revolution  round  the  earth.  After  this  she  will  con- 
tinue to  move  east,  and,  of  course,  she  will  at  dark  be 
on  the  heavens  below  the  eastern  horizon. 

95.  It  will  be  evident  that  she  must  have  passed  the 
point  of  the  ecliptic   180°  from  the  sun,  since  that  point 
is  always  on  the  eastern  horizon  at  sunset.     When  she 
crosses  a  celestial  meridian  running  nearly  north  and 
south  through  this  point,  she  is  said  to  be  in  opposition 
with  the  sun.     She  may  cross  this  line  at  any  time  of 
the  day  or  night,  but  she  does  not  move  fast  enough  to 
get  very  far  from  it  by  the  next  sunset ;  and  so,  when 
she  is  at  opposition,  she  will  appear  to  rise  in  the  east 
about  the  time  the  sun  sets. 

After  opposition  the  moon  can  be  seen  only  by  sit- 
ting up  until  she  rises,  but  her  rising  will  come  later  and 
later,  and  finally  it  will  be  necessary  to  get  out  of  bed 
before  light  in  the  morning  in  order  to  see  her.  After 
her  opposition,  she  approaches  the  sun  on  his  western 
side,  and  she  can  finally  be  found  in  the  morning  near 
him.  Twenty-nine  and  a  half  days  after  opposition, 
she  would  pass  the  sun,  and  after  that  she  would  again 
be  seen  in  the  west  as  new  moon.  Thus,  continued  ob- 

*  The  student  can  readily  understand  the  position  of  these  bodies  in 
nature,  and  therefore  a  diagram  is  quite  unnecessary.  It  is  usually  positively 
pernicious,  since  students  think  of  the  diagram  rather  than  the  positions  in 
nature.  They  may  have  their  heads  filled  with  diagrams  corresponding  to 
nothing  whatever  in  nature  that  they  know. 


THE  MOON  AND  HER  MOTIONS,   AND  HOW   TO   OBSERVE   THEM. 


33 


servation  shows  us  plainly  that  the  moon's  proper  motion 
is  a  revolution  round  the  earth  as  a  center. 

96.  The  Moon's  Orbit. — During  this  revolution  the 
moon  would  not  have  changed  apparent  size,  so  far  as 
the  observer  could  tell  without  measurement.  There- 
fore it  would  be  evident  that  the  figure  of  her  "motion 
was  nearly  a  circle  with  the  earth  as  a  center.*  Meas- 
urement, however,  shows  a  change ;  and  for  this  and 
other  reasons  it  is  believed  that  the  figure  of  her  orbit 
is  an  ellipse.  The  drawings  (Fig.  21)  show  the  variation 
in  the  moon's  apparent  size. 

FIG.  21. 


When  the  moon  is  near  the  horizon,  she  looks  larger 
than  when  she  is  seen  overhead  ;  but  this  is  an  illusion, 
as  is  shown  by  measurement.  We  estimate  her  distance 
from  us  by  so  many  intervening  objects,  that  she  seems 
to  us  farther  off,  and  therefore  larger. 

97.  Sidereal  and  Synodical  Revolutions.  —  At  con- 
junction, or  new  moon,  at  the  beginning  of  these  obser- 
vations, the  moon  would  be  very  near  the  sun ;  and,  as 
the  student  knows,  a  constellation  of  the  zodiac  would 
be  behind,  or  west  of  both.  Thus  the  earth,  moon,  sun, 
and  a  star-group  would  be  nearly  in  line.  After  the 
moon  had  completed  its  revolution  of  twenty-nine  and  a 
half  days,  and  was  again  in  conjunction  with  the  sun, 
and  on  the  western  horizon  at  sunset,  the  same  star- 
group  would  not  be  there,  but  another,  since  the  stars 
would  have  been  moving  west  all  the  time.  The  star- 
group  which  was  on  the  horizon  at  the  first  conjunction 
would  have  gone  below  the  horizon,  and  the  earth  and 
the  moon  would  have  been  in  line  with  the  first  star- 
group  before  the  moon  came  in  line  with  the  earth  and 
sun. 

The  interval  of  time  after  which  the  moon  comes  back 
to  the  line  between  the  earth  and  the  first  star-group 
is  called  a  Sidereal  Revolution  of  the  moon.  The  in- 
terval after  which  the  moon  comes  back  to  the  same 
position  with  regard  to  the  earth  and  sun  is  called  a 
Synodical  Revolution  of  the  moon.  Thus  the  period 
between  two  successive  conjunctions  or  two  successive 

*  That  the  moon's  orbit  is  nearly  a  circle,  follows  from  this,  and  the  fact 
that  her  path  on  the  heavens  is  a  circle. 

s 


oppositions  is  a  synodical  revolution.  A  sidereal  revo- 
lution is  completed  in  twenty-seven  and  one-third  days, 
and  a  synodical  revolution  in  twenty-nine  and  a  half 
days. 

98.  If  the  student  were  asked  why  the  sidereal  revo- 
lution is  shorter  than  the  synodical  revolution,  he  would 
perhaps  say,  "  Because  the  stars  move  west."     But  the 
motion  of  the  stars  is  apparent,  and  due  to  the  earth's 
annual  eastern  motion  round  the  sun.     Therefore  astron- 
omers say  that,  when  the  moon  has  come  round  to  the 
stars  from  which  she  started,  she  has  to  go  a  little  farther 
to  catch  up  with  the  earth  and  revolve  round  the  sun. 

//.   The  Moons  Path  and  the  Ecliptic. 

99.  The  second  line  of  observation  is  to  watch  the 
moon's  path  among  the  stars,  and  learn  how  it  is  situated 
in  regard  to  the  ecliptic.     The  moon  is  nearer  to  us  than 
any  heavenly  body,  while  the  fixed  stars  are  at  an  im- 
measurable distance,  but  she  seems  to  move  among  them, 
because  they  are  on  the  background  ol  the  sphere  against 
which  we  see  her. 

Sometimes  the  moon's  light  obscures  the  stars  near 
her,  so  that  it  is  somewhat  difficult  to  trace  her  path 
with  any  certainty.  But  even  in  this  case  the  student 
must  learn  all  he  can  from  observation,  and  must  not 
stop  because  he  can  not  see  everything. 

100.  The  points  to  be  noted  in  watching  the  moon 
and  the  ecliptic  are  as  follows:  The  student  must  note 
that  the  moon  is  always  in  a  zodiacal  constellation,  and 
always  very  near  the  ecliptic.     She  sometimes  seems  to 
move  toward  it,  sometimes  from  it.     She  crosses  it.     It 
is  somewhat   difficult  to  ascertain   the   exact   point  of 
crossing,  but   the   almanac   gives   help.     The   symbols 
"(§  in  &  "  signify  "  The  moon  to  day  crosses  the  ecliptic 
going  north."    The  symbols  "  ®  in  y  "  signify  "  The  moon 
to  day  crosses  the  ecliptic  going  south." 

101.  The  paths  of  the  sun  and  moon  intersect  at  an 
angle  of  5°.     Therefore  she  is  always  very  near  the  circle 
of  the  ecliptic,  and  when  at  conjunction  she  passes  the 
sun,  she  can  never  be  far  north  or  south  of  the  line  pass- 
ing through  sun  and  earth ;  and,  if  she  is  crossing  the 
ecliptic  at  the  time,  she  will  be  on  it.     Also,  at  opposi- 
tion, she  can  not  be  very  far  north  or  south  of  the  line 
joining  earth  and  sun,  since  that  line  passes  through  the 
point  of  the  ecliptic  180°  from  the  sun.     She  will  be  on 
it  if  she  is  on  the  ecliptic. 

102.  The  points  where  the  path  of  the  moon  crosses 
the  plane  of  the  ecliptic  are  called  Nodes. 

///.    The  Moons  Phases. 

103.  This  is  the  third  subject  to  be  studied  by  the 
observation  of   nature,   beginning  at  new  moon.     Fig. 


34 


ASTRONOMY  BY  OBSERVATION. 


22  represents  the  phases  of  the  moon  from  new  to  full ; 
and,  reversing  the  order,  from  full  to  new  again. 

First  Quarter. — If  we  begin  to  observe  the  new  moon 
as  soon  as  she  is  seen  in  the  west,  she  will  have  the  ap- 
pearance of  i  in  Fig.  22,  viz.,  a  full  circle  covered  with 
a  faint  illumination,  but  having  around  the  margin, 
turned  toward  the  sun  below  the  horizon,  a  slender 
bright  crescent.  If  we  suppose  that  the  moon  shines 
by  the  light  of  the  sun  which  she  reflects  to  us,  and  that 
the  sun  is  a  great  deal  farther  from  us  than  the  moon,  it 
will  account  for  the  appearances.  The  sun  must  en- 
lighten half  the  moon,  and  half  the  moon  must  be  turned 
to  us ;  but  these  must  be  nearly  opposite  halves,  since 
the  moon  is  nearly  between  the  sun  and  earth.  But 
since  the  moon  is  not  on  a  straight  line  between  the 
earth  and  sun,  but  a  little  above  the  line,  it  is  clear  the 
halves  turned  to  us  and  to  the  sun  can  not  be  exactly 
opposite  halves,  but  must  coincide  a  little.  We  see  the 
bright  crescent  because  we  see  a  small  part  of  the  half 
that  is  enlightened  by  the  sun. 

But  the  hemisphere  of  the  moon  turned  toward  the 
earth  just  at  dark  is  turned  toward  the  portion  of  the 
earth  just  below  the  western  horizon,  which  is  bathed  in 
sunshine.  As  moonlight  causes  a  faint  illumination  on 
the  earth,  it  is  clear  that  sunlight  reflected  from  the  earth 
to  the  moon  could  cause  the  faint  light  seen  over  the 
moon's  whole  surface. 

Second  Quarter. — If  we  continue  to  observe  the 
moon,  we  shall  see  that,  as  she  moves  east,  she  rises 
higher,  and  the  crescent  increases ;  and  at  seven  days 
after  new  moon  she  is  overhead  at  sunset  and  is  a  half 
moon.  She  has  then  passed  her  first  quarter,  as  it  is 
called,  and  begins  the  second.  In  this  case,  her  western 
side  is  still  turned  toward  the  sun ;  but,  since  she  is 
above  our  heads  at  sunset,  only  half  the  western  hemi- 
sphere coincides  with  the  hemisphere  which  we  see 
above  us.  For  this  reason  we  find  only  a  half-hemi- 
sphere enlightened  or  visible,  and  the  convex  side  of 
that  is  turned  toward  the  west. 

Third  Quarter. — After  this,  as  the  moon  continues  to 
move  east,  she  increases  in  size,  and  about  fifteen  days 


after  new  moon  she  begins  her  third  quarter.  She  is 
then  full,  or  shows  a  full  enlightened  circle ;  and  she  is 
rising  in  the  east  while  the  sun  is  setting  in  the  west. 
The  same  side  is  now  turned  toward  us  and  toward  the 
sun,  for  she  is  now  nearly  in  line  with  sun  and  earth, 
the  earth  being  in  the  middle.  If  she  were  quite  in  line, 
it  is  evident  that  the  earth  would  cut  off  the  sun's  light 
from  her,  and  we  should  have  an  eclipse  of  the  moon. 
If  at  full  moon  she  is  in  that  part  of  her  path  on  the 
heavens  which  crosses  the  ecliptic,  she  is  in  a  direct  line 
with  earth  and  sun. 

At  full  moon  she  is  180°  from  the  sun  on  the  celestial 
sphere  ;  and,  as  she  still  moves  east,  she  must  approach 
the  sun  on  the  other  side,  and  so  she  begins  to  decrease 
on  her  western  side.  After  the  beginning  of  her  third 
quarter,  she  can  generally  be  seen  in  the  daytime,  be- 
cause she  is  so  far  from  the  sun  on  the  sphere  that  he 
does  not  entirely  obscure  her  light.  When,  from  ap- 
proaching the  sun,  she  becomes  invisible  in  the  daytime, 
she  can  best  be  observed  by  rising  just  before  light  in 
the  morning.  We  know  that,  at  that  hour,  the  sun  is 
just  below  the  eastern  horizon. 

Fourth  Quarter. — About  twenty-one  days  after  new 
moon,  she  begins  her  fourth  quarter.  She  is  then  a  half- 
moon  again,  and  when  seen  while  it  is  still  dark  in  the 
morning,  she  is  nearly  overhead,  with  her  convex  side 
turned  toward  the  sun.  It  is  evident  that  he  illuminates 
her  eastern  hemisphere,  and  that  we  see  only  half  of  it. 
The  half-hemisphere  which  we  do  see  has  its  convex 
side  turned  east. 

After  this,  we  should  find  the  moon  drawing  nearer 
the  sun  at  sunrise,  and  about  the  twenty-seventh  day 
after  new  moon  we  should  again  see  the  faintly  illumined 
full  circle  with  the  bright,  slender  crescent  on  the  side 
next  the  sun.  Sun  and  earth  would  again  face  opposite 
sides  of  the  moon,  nearly,  but  not  exactly.  The  moon 
being  a  little  above  the  line  between  earth  and  sun,  the 
hemisphere  opposite  each  would  to  a  very  small  extent 
coincide ;  and  this  is  the  reason  why  we  should  see  the 
crescent.  The  faint  illumination  would  be  turned  to- 
ward the  sunny  side  of  the  earth  below  the  eastern  hori- 


THE  MOON  AND  HER  MOTIONS,   AND  HOW   TO   OBSERVE    THEM. 


35 


zon.  Finally,  the  moon  would  pass  nearly  between  earth 
and  sun,  and  then  we  should  have  new  moon  again.  If 
the  moon  passed  directly  between  earth  and  sun,  it  is 
clear  she  would  intercept  the  sun's  light  from  us,  and 
we  should  have  an  eclipse  of  the  sun.  There  is  no 
eclipse  unless  moon  and  sun  are  together  at  the  points 
where  the  moon's  path  on  the  heavens  crosses  the 
ecliptic. 

The  moon's  phases  are  sometimes  illustrated  by  a 
diagram  which  is  given  below  (Fig.  23),  mainly  because 
some  teachers  will  like  to  have  it.  The  knowledge  which 


The  Phases  of  the  Moon. 

such  a  diagram  seems  to  give  is  very  delusive.  The 
student  will  be  very  unwise  not  to  watch  the  moon 
herself. 

From  observing  the  moon  it  is  very  clear  that  she 
shines  by  light  reflected  from  the  sun,  for  we  do  not  see 
any  illumination  except  on  the  parts  which  are  turned 
toward  the  sun. 

104.  Nothing  can  give  a  student  of  astronomy  a  com- 
plete idea  of  the  causes  and  conditions  of  eclipses  but 
observation  of  the  relative  positions  of  sun,  moon,  and 
earth  through  a  full  synodical  revolution.  He  will  see 
that  sun,  moon,  and  earth  never  are  nearly  in  line  except 
at  new  and  full  moons ;  that  at  new  moon  only,  they  are 
nearly  in  line  with  the  moon  in  the  middle ;  and  there- 
fore an  eclipse  of  the  sun  can  take  place  at  new  moon 
alone.  Also,  at  full  moon  only,  they  are  nearly  in  line 
with  the  earth  in  the  middle,  and  therefore  an  eclipse  of 
the  moon  can  take  place  at  full  moon  alone.  He  will 
see  that  they  never  can  be  in  line  unless  the  moon  at 
full  or  new  is  on  the  ecliptic ;  that  is,  unless  the  earth 
and  moon  are  on  the  line  in  which  the  planes  of  their 
orbits  intersect. 


The  subject  of  eclipses  will  be  more  fully  treated 
further  on. 

105.  Positions  of  the  Moon's  Crescent. — From  new 
to  full,  the  moon  increases  gradually  from  a  crescent  to 
a  full  circle,  and  back  again  from  full  to  new  ;  but,  owing 
to  the  fact  that  the  moon  is  sometimes  north  of  the 
ecliptic,  sometimes  south  of  it — that  is,  sometimes  north 
of  the  sun's  path,  and  sometimes  south  of  it — the  posi- 
tions of  the  crescent  vary.  The  variation  is  most  noticed 
at  new  moon,  when  the  sun  is  known  to  be  just  below 
the  horizon.  Sometimes  a  line  joining  the  horns  is  nearly 
vertical,  as  f);  sometimes  it  is  nearly  horizontal,  as  ©. 
Superstitious  people  call  the  first  "  wet  moon,"  and  the 
last  "  dry  moon,"  and  suppose  they  foretell  the  weather. 
The  variation  really  depends  on  the  positions  of  the  moon 
and  sun  in  regard  to  the  horizon. 

The  moon's  crescent  may  have  any  position  inter- 
mediate between  these.  It  is  most  nearly  horizontal  at 
the  new  moon  near  March  2ist,  and  most  nearly  perpen- 
dicular at  the  new  moon  near  September  2ist.  At  sun- 
set on  March  2ist  the  ecliptic  has  nearly  the  aspect  of 
Map  I.  Fig.  24  shows  the  western  hemisphere  of  the 
heavens  at  this  time,  and  it  shows  that  the  ecliptic  is 


nearly  perpendicular  to  the  horizon.  In  this  case,  the 
sun,  which  is  always  on  the  ecliptic,  is  nearly  below  the 
intersection  of  ecliptic  and  horizon.  If  the  moon  is  at 
the  same  time  north  of  the  ecliptic,  she  is  directly  over 
the  sun,  and  as  the  convex  side  of  the  crescent  is  always 
turned  toward  the  sun,  it  is  nearly  horizontal. 

On  September  2ist,  at  sunset,  the  ecliptic  has  the 
aspect  of  that  circle  on  Map  II.  Fig.  25  shows  the  west- 
ern hemisphere  of  the  heavens  at  sunset  on  September 
2 ist,  and  it  is  seen  that  the  ecliptic  is  much  inclined  to 


ASTRONOMY  BY  OBSERVATION. 


the  horizon.     The  sun,  after  setting,  is  far  north  of  the 
intersection  of  ecliptic  and  horizon.     If  the  moon  at  this 


time  is  south  of  the  ecliptic,  she  is  nearly  south  of  the 
sun,  and  therefore  the  line  joining  the  horns  of  the  cres- 
cent is  nearly  vertical. 

The  facts  which  explain  these  positions  of  the  cres- 
cent are:  I.  The  position  of  the  moon  in  relation  to  the 
ecliptic.  2.  The  two  positions  of  the  ecliptic  in  relation 
to  the  horizon.  These  facts  will  have  little  reality  to 
the  student  unless  he  sees  them  in  nature.  But  in  order 
to  give  them  reality,  it  is  not  necessary  for  him  to  wait 
and  see  them  with  the  new  moon. 

106.  The  Moon's  Axial  Rotation. — The  most  careless 
person  usually  remembers,  without  special  observation, 
that  the  moon  always  presents  the  same  shadings  of  sur- 
face, in  which  many  persons  have  traced  a  resemblance 
to  a  man's  face.     Thus  it  is  evident  that  we  must  see 
nearly  the  same  hemisphere  of  the  moon  all  the  time. 
If  the  student  will  walk  around  a  chair  with  his  face 
turned  to  it  all  the  time,  he  will  imitate  the  motion.     In 
doing  this  he  turns  his  face  once  to  every  point  of  the 
compass.     This  is  precisely  what  he  does  when  he  stands 
on  one  spot  and  turns  round  or  revolves  axially.     There- 
fore the  moon's  motion  is  usually  described  by  saying 
that  she  revolves  once  on  her  axis  while  performing  her 
revolution  in  her  orbit. 

107.  The  Moon's  Librations. — At  different  times  we 
really  see  a  little  more  than  one  half  the  moon's  surface. 
The  motions  to  which  this  is  due  are  called  the  moon's 
Librations.     In  consequence  of  the  elliptical  form  of  the 
moon's  orbit,  her  motion,  like  that  of  the  earth,  is  un- 
equal.    But  her  motion  on  her  axis  is  equal,  and  there- 
fore her  orbital  motion  sometimes  gets  ahead  of  her 


axial  motion,  or  falls  behind  it,  and  thus  we  see  a  little 
farther  around  her  east  or  west.  This  is  called  her 
Libration  in  Longitude. 

The  moon's  axis,  like  that  of  the  earth,  is  inclined  to 
the  plane  of  her  orbit,  and,  like  the  earth's  axis,  always 
moves  parallel  to  itself.  We  know  this,  because,  when 
we  see  the  moon  through  a  telescope,  she  shows  a  change 
in  the  part  round  the  poles.  This  is  called  the  moon's 
Libration  in  Latitude. 

Owing  to  the  fact  that  we  are  about  four  thousand 
miles  from  the  center  of  the  earth,  which  is  the  center 
of  the  moon's  motion,  we  see  her  from  slightly  different 
positions  when  on  our  eastern  and  western  horizons  ;  and 
there  is  thus  a  slight  variation  in  the  part  of  the  surface 
seen.  This  is  called  her  Parallactic  Libration.  Parallax 
is  the  displacement  of  an  object  caused  by  the  observer's 
change  of  position. 

We  see  in  all  about  .58  of  the  moon's  surface. 

108.  Times  of  the  Moon's  Risings. — What  we  call 
the  moon's  rising  is  due  to  the  earth's  rotation  on  her 
axis,  carrying  the  horizon  east  to  meet  her.     But  during 
one  rotation  of  the  earth  the  moon  moves  13°  east  in  her 
orbit,  and  therefore  the  horizon  must  move  farther  to 
overtake  her.     As  the  earth  and  horizon  revolve  through 
i°  in  four  minutes,  it  takes  fifty  minutes,  on  an  average, 
for  the  horizon  to  catch  up  with  the  moon.     Therefore 
she  rises  about  fifty  minutes  later  every  day.     But  the 
times  of  the  moon's  rising  vary  a  good  deal. 

109.  Harvest  Moon. — When  the  constellation  Pisces  is 
on  the  eastern  horizon  at  the  time  the  moon  rises,  the 
retardation  in  her  rising  on  successive  nights  is  much 
lessened.     In  latitude  40°  the  delay  may  be  only  about 
twenty-five  minutes.     The  difference  is  the  more  noticed 
when  the  moon  is  full,  both  because  she  is  then  more 
conspicuous,  and  because  she  rises  at  a  more  convenient 
hour  for  observation.     The  full  moon  rises  in   Pisces 
within  a  fortnight  of  the  autumnal  equinox  in  Septem- 
ber.    It  is  widely  noticed  by  farmers,  to  whom  its  early 
risings  are  of  use  in  gathering  the  crops.     They  call  it 
Harvest  Moon. 

When  Virgo  is  on  the  eastern  horizon  at  full  moon, 
which  happens  within  a  fortnight  of  the  vernal  equinox 
in  March,  the  delay  in  her  rising  is  increased.  In  lati- 
tude 40°  the  difference  in  her  time  of  rising  on  successive 
nights  may  equal  an  hour  and  a  quarter. 

An  explanation  (and  diagrams)  of  these  phenomena 
are  of  little  use  except  to  aid  the  student  in  observing 
the  facts  in  nature  which  cause  them.  A  knowledge  of 
mere  diagrams  is  illusory.  In  order  to  have  real  knowl- 
edge, however,  it  is  not  necessary  to  see  all  the  facts 
together,  and  therefore  not  necessary  to  wait  until  har- 
vest moon.  The  facts  are  as  follows:  i.  The  moon  is 


THE  MOON  AND  HER   MOTIONS,   AND  HOW   TO   OBSERVE    THEM. 


37 


always  very  near  the  ecliptic,  and  we  can  tnerefore  use 
the  ecliptic  to  illustrate  the  angle  which  her  path  among 
the  stars  makes  with  the  horizon.  2.  The  moon  moves 
east  among  the  stars  ;  this  causes  the  delay  in  her  ris- 


FIG.  26. 


ing.  3.  In  March  the  ecliptic  (and  the  moon's  path) 
seem  to  curve  very  little  toward  the  south,  and  she  there- 
fore moves  on  a  path  nearly  perpendicular  to  the  hori- 
zon. Map  I  shows  this,  and  so  does  a'.so  Fig.  26,  exhibit- 
ing the  eastern  hemisphere  of  the  heavens  and  the  aspect 
of  the  ecliptic  in  March.  Also,  in  September,  the  ecliptic 
is  much  inclined  to  the  horizon.  This  is  shown  in  Map 
II  and  in  Fig.  27,  exhibiting  the  eastern  hemisphere  of 
the  heavens  for  September,  with  the  aspect  of  the  ecliptic. 
It  is  clear  that  it  would  take  longer  to  overtake  the  moon 

FIG.  27. 


'HERE 


moving  away  from  the  horizon  on  a  path  like  the  ecliptic 
of  Fig.  26,  than  when  she  moves  on  a  path  like  the  ecliptic 


of  Fig.  27,  even  if  she  moved  13°  daily  on  both.  And  as 
her  rising  is  due  to  the  horizon  moving  down  to  over- 
take her,  she  is  less  delayed  in  rising  on  successive 
nights  in  September ;  and  in  March  the  delay  is  greater. 
In  December  and  June,  the  path  of  the  ecliptic  is  nearly 
parallel  with  the  equinoctial,  and  the  angle  of  the  equi- 
noctial and  the  horizon  does  not  vary  at  all,  as  can  be 
seen  from  Figs.  26  and  27. 

Since  the  ecliptic  is  not  a  visible  line  in  nature,  it  can 
not  be  traced  so  definitely  as  on  the  diagram ;  but  a 
careful  observer,  tracing  it  by  the  stars,  and  noting  the 
direction  of  the  zodiacal  constellations,  will  have  no 
difficulty  in  seeing  the  difference  in  the  angle,  as  ex- 
hibited in  March  and  September. 

Harvest  moon  has  these  peculiarities  only  in  high 
latitudes,  because  the  great  variation  in  the  angles  is  due 
to  the  elevation  of  the  pole  above  the  horizon.  The 
elevation  of  the  pole  makes  all  circles  running  east  and 
west  (or  nearly  east  and  west)  curve  southward  at  and 
near  the  point  where  they  cross  the  meridian  above  us. 
A  consideration  of  Figs.  26  and  27  will  show  that  the 
difference  in  the  angles  with  the  horizon  is  due  to  the 
elevation  of  the  pole  above  the  horizon. 

no.  The  Moon's  Motion  North  and  South. — Since 
the  moon's  path  on  the  heavens  is  so  near  the  ecliptic,  she 
must  vary  in  position  north  and  south.  Also,  since  she 
goes  through  180°  in  about  fifteen  days,  she  must  move 
from  north  to  south  much  faster  than  the  sun.  This  can 
be  seen  plainly  by  watching  her  risings  and  settings  on 
successive  evenings.  Since  full  moons  are  always  180° 
from  the  sun,  they  are  far  north  in  winter,  and  longer 
above  the  horizon  than  in  summer,  when  they  are  far 
south.  Thus  they  illumine  the  long  winter  nights  when 
they  are  most  needed. 

in.  Revolution  of  the  Nodes. — Let  us  suppose  that 
the  student,  watching  the  moon,  sees  her  approaching  the 
ecliptic,  and,  aided  by  the  entries  of  the  almanac,  "®  in 
&,"  and  "(1  in  y,"  identifies  with  some  degree  of  ac- 
curacy a  point  where  she  crosses  the  ecliptic.  If,  a  year 
or  eighteen  months  afterward,  he  observes  the  moon  at 
the  same  crossing,  he  will  find  that  she  crosses  earlier, 
or  farther  west.  Of  course,  the  intersections,  moving  on 
a  circle,  will  finally  come  round  to  the  point  from  which 
they  started.  Now  the  moon  seems  to  cross  the  ecliptic 
on  the  heavens,  only  because  she  is  then  crossing  the 
plane  of  the  ecliptic.  Thus  this  revolution  shows  that 
the  moon's  nodes  revolve  on  her  orbit.  It  is  completed 
in  about  nineteen  years. 

Eclipses. 

112.  Umbra  and  Penumbra. — When  a  luminous  body 
is  larger  than  a  point,  the  shadows  made  by  intercepting 


ASTRONOMY  BY  OBSERVATION. 


FIG.  28. 


its  light  consist  of  two  portions.  There  is  a  dark  central 
part  which  receives  no  direct  rays  from  the  luminary. 
This  is  called  the  Umbra.  Surrounding  the  umbra, 
there  is  a  fringe  of  less  dense  shadow,  which  receives 
direct  rays  from  some  portions  of  the  luminous  body, 
but  not  from  all  portions.  This  is  called  the  Penumbra. 

113.  Shadows  of  Earth   and  Moon. — The  shadows 
formed   by  intercepting   the   sun's   light  consist  of  an 
umbra  and  a  penumbra.     The  umbra  belonging  to  the 
shadow  of  the  earth  or  moon  is  cone-shaped,  with  the 
apex  of  the  cone  turned  away  from  the  sun.     The  pe- 
numbra increases  in  width  with  the  distance  from  the 
sun  (see  Fig.  28). 

114.  Eclipses  of  the  Moon.-f  An  eclipse  of  the  moon 

takes  place  when- 
ever the  moon  passes 
through  the  umbra 
of  the  earth's  shadow. 
So  small  an  obscu- 
ration results  from 
the  moon's  passage 
through  the  penum- 
bra, that  it  is  not 
called  an  eclipse.  In 
a  total  eclipse  of  the 
moon  her  whole 
sphere  is  still  faintly 
visible,  and  is  of  a 
coppery  hue.  This  is 
due  to  rays  of  light 
refracted  by  the 
earth's  atmosphere, 
so  as  to  fall  on  the 
moon. 

115.  Eclipses  of 
the  Sun.-f-A  total  so- 
lar eclipse  takes  place 
in  any  part  of  the 
earth  which  is  in  the 
umbra  of  the  moon's 
shadow./  The  umbra 
of  the  moon's  shad- 
ow is  so  small  that  it 
does  not  often  cause 
darkness  over  a  part 
of  the  earth's  surface 
more  than  a  hundred 
miles  in  diameter,  but 

Explanation  of  Solar  and  Lunar  Eclipses.        bJ   the    earth's   rapid 

rotation,  this  shadow 

is  carried  forward  over  a  long  zone  or  band  of  territory 
about  a  hundred  miles  in  width.  A  total  solar  eclipse 


is  a  very  striking  event.  The  stars  come  out  and  the 
animals  go  to  rest.  A  total  eclipse  of  the  sun  is  an  inter- 
esting event  to  astronomers,  because  it  gives  them  an 
opportunity  of  studying  the  sun's  corona,  of  which  an 
account  will  be  given  in  the  chapter  on  "  The  Sun." 

A  partial  solar  eclipse  exists  in  those  parts  of  the 
earth  which  are  in  the  penumbra  of  the  moon's  shadow. 

The  moon's  distance  from  the  earth  varies,  on  ac- 
count of  the  elliptical  form  of  her  orbit.  If  an  eclipse  of 
the  sun  occurs  when  the  moon  is  at  her  greatest  distance 
from  the  earth,  her  apparent  diameter  is  diminished,  and 
she  can  not  entirely  hide  the  sun  from  us.  We  have 
then  what  is  called  an  Annular  Eclipse  of  the  Sun.  The 
moon  conceals  the  central  part  of  the  sun's  surface,  but 
around  her  dark  body  is  seen  a  ring  of  brilliant  light. 

A  total  eclipse  of  the  sun  lasts  but  a  few  minutes,  and 
often  only  a  few  seconds.  A  partial  eclipse  lasts  several 
hours.  A  lunar  eclipse  may  be  total  for  two  hours,  dur- 
ing which  time  the  moon  is  crossing  the  umbra  of  the 
earth's  shadow. 

116.  Frequency  of  Eclipses. — Solar  eclipses  are  more 
frequent  than  lunar  eclipses.     The  reason  can  be  seen 
by  examining  Fig.   28.      In  a  lunar  eclipse  the   moon 
crosses  the  cone  of  the  earth's  umbra.     In  a  solar  eclipse 
she  crosses  a  prolongation  of  the  same  cone  toward  the 
sun,  which  is  equal  to  a  broader  part  of  the  cone.     But 
in  any  one  place  there  are  more  lunar  than  solar  eclipses, 
because  a  lunar  eclipse  is  seen  wherever  the  moon  is 
visible,  while  a  solar  eclipse  is  visible  over  a  small  ter- 
ritory.    A  total  or  annular  eclipse  of  the  sun  is,  in  any 
one  place,  an  event  of  very  rare  occurrence.     There  has 
been  no  total  solar  eclipse  in  London  since   1715,  and 
there  was  none  for  more  than  five  hundred  years  before 
that  date. 

117.  Causes  of  Eclipses. — Whenever  the  sun  or  moon 
is  eclipsed,  some  parts  of  the  sun,  moon,  and  earth  must 
be  in  line.     It  is  evident  they  can  not  be  in  line,  with  the 
moon  in  the  middle,  except  at  conjunction,  and  therefore 
a  solar  eclipse  must  take  place  at  the  passage  of  the 
moon  from  old  to  new.     The  sun,  moon,  and  earth  can 
not  be  in  line,  with  the  earth  in  the  middle,  except  at 
opposition,  and  therefore  a  lunar  ech'pse  must  take  place 
at  full  moon. 

But  opposition  and  conjunction  do  not  always  bring 
eclipses.  The  reason  is  evident :  the  moon  is  not  always 
at  the  intersection  of  her  orbit  with  the  ecliptic  when 
she  is  at  conjunction  or  opposition. 

The  moon  passes  the  nodes,  or,  in  other  words,  crosses 
the  ecliptic,  during  every  revolution  round  the  earth  ; 
and  yet  there  is  not  always  an  eclipse.  But  the  reason 
of  this  is  also  plain :  the  sun  and  earth  are  not  always 
in  line  with  a  node  when  she  passes  it. 


THE  PLANETS  AND    THEIR  MOTIONS,   AND   HOW   TO   OBSERVE   THEM. 


39 


The  moon's  path  on  the  heavens  crosses  the  ecliptic 
at  an  angle  of  5°.  In  other  words,  the  plane  of  her  orbit 
and  the  plane  of  the  earth's  orbit  intersect  at  an  angle  of 
5°.  It  is  evident,  from  Fig.  29,  that  some  parts  of  the 

FIG.  29. 


earth  and  moon  may  be  in  line,  though  not  their  centers, 
both  just  before  and  just  after  she  passes  a  node. 

A  line  joining  the  points  where  the  moon's  path  on 
the  heavens  crosses  the  ecliptic,  passes  through  the 
nodes  of  the  moon's  orbit.  When  the  earth  and  sun 
cross  that  line,  twice  a  year,  they  "  pass  the  nodes." 
Students  generally  understand  the  circumstances  of 
eclipses  better  by  studying  a  few  records  of  eclipses 
from  almanacs  of  successive  years : 

1882. — May  1 7th,  solar  eclipse  ;  November  nth,  solar 
eclipse. 

1883. — April  22d,  lunar  eclipse;  May  6th, solar  eclipse; 
October  I5th-i6th,  lunar  eclipse;  October  3oth,  solar 
eclipse. 

1884. — March  2/th,  solar  eclipse;  April  loth,  lunar 
eclipse;  April  25th,  solar  eclipse;  October  4th,  lunar 
eclipse  ;  October  i8th,  solar  eclipse. 

1885. — March  1 6th,  solar  eclipse;  March  3oth,  lunar 
eclipse ;  September  8th,  solar  eclipse ;  September  23d, 
lunar  eclipse. 

From  these  records  the  student  sees  there  are  two 
eclipse  periods  in  every  year — evidently  at  the  passage 
of  the  nodes  by  the  earth  and  sun.  These  periods  come 
earlier  every  year,  a  result  plainly  due  to  the  backward 
movement  of  the  nodes  on  the  ecliptic.  The  record  of 
1883  can  be  explained  by  remembering  that  the  earth 
and  sun  move  very  slowly  from  the  node,  while  the 
moon  crosses  the  heavens  in  a  little  less  than  fifteen 
days.  In  1883  the  earth  remained  at  the  nodes  long 
enough  for  the  moon  to  be  eclipsed  and  then  cross  the 
heavens  to  eclipse  the  sun  fourteen  or  fifteen  days  after- 
ward. The  record  of  1884  shows  that  the  moon  may 
partially  eclipse  the  sun  on  one  side,  cross  the  heavens 
to  be  eclipsed  herself  fourteen  days  afterward,  and  then 
cross  back  to  eclipse  the  sun  twenty-nine  days  after- 
'  ward. 

The  student  will  not  be  surprised  to  learn  that  the 
slow  motion  of  the  sun  and  earth,  together  with  the 
rapid  motion  of  the  moon,  renders  it  impossible  for  the 
two  former  bodies  to  pass  the  moon's  nodes  without  an 
eclipse. 


From  the  record  of  1882  it  is  clear  that  there  is  a 
period  of  less  than  six  months  between  two  passages  of 
the  nodes  by  the  sun  and  earth.  Therefore,  if  this  event 
takes  place  just  at  the  beginning  of  the  year,  they  will 
pass  again  in  less  than  six  months, 
and  will  thus  reach  the  beginning 
of  the  third  eclipse  period  within 
the  year.  When  there  are  seven 
eclipses  in  a  year,  four  must  be 
°°N's  PATH  —  eclipses  of  the  sun.  If  there  are 

but  two,  both  are  eclipses  of  the  sun. 
There  can  not  be  more  than  seven  or  fewer  than  two. 

A  further  account  of  the  moon  will  be  given  in  the 
General  Account  of  the  Solar  System,  Part  II. 


CHAPTER    VI. 

THE    PLANETS    AND    THEIR    MOTIONS,    AND    HOW    TO 
OBSERVE    THEM. 

118.  The  planets  are  stars  which,  like  the  earth,  re- 
volve round  the  sun  as  a  center.     Five  of  these  bodies, 
Mercury,  Venus,   Mars,  Jupiter,   Saturn,    can    be   seen 
without  a  telescope ;  and  the  first  four  are  so  often  in 
the  sky  above  us  just  after  dark,  that  there  is  no  diffi- 
culty in  knowing  them  by  sight,  and  in  seeing  and  un- 
derstanding their  motions. 

It  is  the  object  of  this  chapter  to  give  such  an  intel- 
ligible account  of  these  movements,  in  the  order  in  which 
an  observer  will  see  them,  that  any  sensible  student  may 
use  it  to  gain  what  will  be  a  life-long  pleasure  and  ad- 
vantage, viz.,  the  power  to  look  at  the  changes  in  the 
heavens  with  intelligent  recognition. 

119.  The  Superior  Planets. — For  reasons  which  will 
be  made  clear  a  little  further  on,  the  planets  are  divided 
into  two  classes,  called  Superior  and  Inferior  Planets. 
Jupiter,  Saturn,  and  Mars  are  the  superior  planets  which 
can  be  seen  with  the  unaided  eye. 

120.  How  to  find  Jupiter,  Mars,  or  Saturn. — Except 
when  they  are  very  near  the  sun,  Jupiter,  Mars,  and  Sa- 
turn can  be  seen  at  some  hour  of  every  clear  night.     The 
student's  knowledge  of  them  will  probably  be  chiefly 
gained  by  watching  them  in  the  evening  before  bed- 
time, and  it  is  well  to  form  a  habit  of  observing  them  at 
dark.     These  planets  come  into  the  sky  from  the  east. 
It  will  give  reality  to  the  study  of  them  if  the  student 
who  begins  this  chapter  will  get  a  good  almanac,  and 
find   where  they  are  then,  or  when  they  will  become 
evening  stars.     Three  steps  are  to  be  taken  :   i.  To  find 
out  when  they  are  evening  stars.     2.  The  student  must 


ASTRONOMY  BY  OBSERVATION. 


know  where  in  the  heavens  to  look  for  them.  3.  He 
must  understand  how  they  can  certainly  be  known  when 
seen.  A  few  words  will  be  said  on  each  subject : 

121.  I.  How  to  tell  when  a  Planet  is  in  the  Evening 
Sky. — They  come  in  at  opposition,  they  go  out  at  conjunction. 
The  dates  of  the  oppositions  of  Jupiter,  Mars,  and  Saturn  are 
given  in  any  good  astronomical  almanac  under  the  following 
symbols;    8  means  opposition;   O,  the  sun;    8  K  O,  opposi- 
tion of  Jupiter  with  the  sun  ;    8  *?  O,  opposition  of  Saturn  with 
the  sun  ;    8  $  O,  opposition  of  Mars  with  the  sun. 

The  publishers  of  this  book  will  send  out  with  every  copy 
sold  a  printed  slip  containing  any  oppositions  coming  within 
three  years  from  the  current  year.  The  oppositions  of  Saturn 
come  once  in  about  a  year ;  those  of  Jupiter,  every  thirteen 
months  ;  those  of  Mars,  every  twenty-six  months.  About  two 
months  before  opposition,  a  superior  planet  can  be  seen  in  the 
evening  as  early  as  ten  o'clock.  For  this  reason  it  is  best  to 
begin  observation  about  two  months  before  opposition.  The 
planets  are  not  called  "  evening  stars  "  until  they  rise  at  sunset ; 
and  they  are  called  "  morning  stars  "  from  the  time  that  they 
rise  with  the  sun.  The  best  almanacs  give  the  times  of  risings 
of  the  planets. 

122.  II.  In  what  Part  of  the  Sky  must  these  Planets 
be  looked  for? — They  are  always  found  in  the 

zodiacal  constellations,  and  very  near  the  ecliptic. 

They  are  never  more  than  2}/s°  from  the  ecliptic.       ^ 

The  student  who  knows  the  constellations  of  the 
zodiac,  can  tell  almost  at  a  glance  whether  Saturn,  Jupiter, 
or  Mars  is  visible.  A  star  of  the  first  magnitude  seen  where 
one  was  never  seen  before,  by  a  person  familiar  with  the  eclip- 
tic, is  sure  to  be  a  planet.  Intelligent  acquaintance  with  the 
heavens  is  not  possible  without  the  knowledge  of  the  ecliptic. 
In  the  introduction  to  this  book,  "  Directions  how  to  learn  and 
teach  the  Constellations,"  will  be  found  careful  directions  for 
learning  the  ecliptic.  All  the  zodiacal  constellations  visible  at 
one  time  can  be  learned  in  a  few  evenings. 

It  is  perhaps  well  to  add  a  few  words  about  each  superior 
planet,  and  to  say  that  they  can  be  most  easily  identified  at  and 
near  opposition,  both  because  they  are  then  brightest,  and  be- 
cause we  know  we  must  look  for  them  at  dark  not  far  from  the 
eastern  horizon.  Jupiter  can  be  recognized  at  any  time  of 
night  when  he  is  known  to  be  above  the  horizon,  because  he 
will  be  the  brightest  star  visible  unless  Venus  is  present ;  and 
he  can  easily  be  distinguished  from  her  by  the  test  of  planetary 
motion,  which  will  presently  be  explained  under  III. 

Near  opposition,  Mars  will  be  as  bright  as  any  star  in  the 
east  except  Jupiter.  It  is  easy  to  know  whether  Jupiter  is 
in  the  sky,  and  then  Mars  can  be  distinguished  by  his  red  color. 
Mars  varies  in  luster  more  than  any  of  the  three,  and  could 
hardly  be  identified  by  an  inexperienced  observer  when  he  is 
more  than  eight  months  from  opposition. 

Saturn  varies  less  than  any  of  the  three.  An  observer  must 
find  him  chiefly  by  knowing  the  zodiacal  stars  near  the  ecliptic, 
and  thus  recognizing  a  stranger  among  them.  He  is  about  as 
bright  as  the  brighter  first-magnitude  stars. 


123.  III.  How  the  Planets  can  certainly  be  known 
when  seen. — When  the  observer  thinks  he  has  found  a  planet, 
or  wishes  to  distinguish  between  two  or  three  stars,  one  of  which 
he  supposes  to  be  some  planet,  he  can  make  perfectly  sure  by 
the  test  of  planetary  motion  now  to  be  described.  The  stars 
are  divided  into  planets  and  fixed  stars.  The  fixed  stars  can 
never  be  seen  to  approach  or  recede  from  one  another  except 
by  trained  observers  using  instruments  of  the  most  delicate  and 
extreme  accuracy.*  For  this  reason  the  constellations  keep  the 
same  figures,  coming  and  going  as  if  painted  on  a  rolling  pano- 
rama. But  the  planets  recede  from  one  star  and  approach 
another.  Our  study  of  them  consists  chiefly  in  watching  this 
motion.  It  can  not  be  done  without  care  and  patience,  but, 
when  the  student  gets  fairly  at  it,  it  becomes  very  interesting 
work. 

When  the  student  first  sees  a  planet,  or  a  star  supposed  to 
be  one,  he  notes  very  carefully  its  position  in  regard  to  fixed 
stars  near  it.  Very  often  it  will  be  found  in  line  with  two  other 
stars,  as  seen  in  Fig.  30. 

In  this  case  any  movement  will  at  once  be  indicated  by  the 
planet  being  out  of  the  line  (breaking  line),  unless  the  line  is 
nearly  parallel  with  the  ecliptic.  Sometimes  it  forms  regular 
figures,  as  triangles,  right  or  isosceles,  and  then  movement  shows 

FIG.  30. 


itself  very  soon  by  the  alteration  of  the  figure.  The  motion  of 
Mars  can  usually  be  detected  in  forty-eight  hours ;  that  of 
Jupiter  in  a  week  or  less.  Jupiter  and  Venus  can  be  readily 
distinguished  from  one  another  by  the  rapidity  of  the  move- 
ment of  the  latter,  which  becomes  evident  after  an  interval  of 
twenty-four  hours.  Saturn  moves  more  slowly  than  any  of  the 
planets  seen  with  the  unaided  eye  ;  but  the  movement  can  al- 
ways be  detected  when  his  position  in  relation  to  other  stars 
is  well  observed. 

All  this  observation  requires  perseverance,  but  the  student 
should  be  encouraged  by  knowing  that  it  affords  valuable  train- 
ing for  a  faculty  much  neglected  in  our  schools,  viz.,  the  power 
of  intelligent  observation.  As  fast  as  a  planet  moves  out  of  one 
figure  that  he  has  discovered,  he  seeks  to  find  another.  He 
notes  the  direction  of  the  motion,  and  whether  the  planet  is 
north  or  south  of  the  ecliptic.  A  star  which  does  not  move 
among  the  stars  can  not  be  a  planet. 

124.  Motions  and  Appearances  of  Superior  Planets. 
—There  are  two  important  motions  which  are  the  key 

to  our  knowledge  of  a  superior  planet.  They  are  called 
the  Synodical  Revolution  and  the  Sidereal  Revolution. 

125.  The  Synodical  Revolution. — Let  us  suppose  the 
observer  begins  at  the  planet's  opposition  with  the  sun. 
It  will  be  found  at  dark  just  above  the  eastern  horizon  ; 
and,  as  the  sun  is  known  to  be  just  below  the  western 

*  The  trained  observers  have  detected  the  motion  of  only  a  few. 


THE  PLANETS  AND    THEIR  MOTIONS,   AND  HOW   TO   OBSERVE   THEM. 


horizon,  the  planet  is  seen  to  be  opposite  the  sun.  If  it 
is  on  the  ecliptic,  it  is  in  a  straight  line  with  earth  and 
sun ;  but  since  it  is  always  on  or  very  near  the  ecliptic, 
it  is  always  very  nearly  in  line  at  opposition. 

Since  the  planets,  like  all  other  heavenly  bodies,  have 
an  apparent  diurnal  revolution  owing  to  the  earth's  rota- 
tion upon  her  axis,  we  must,  to  avoid  confusion,  watch 
the  synodical  revolution  at  the  same  hour  of  the  even- 
ing. Some  time  about  dark  is  most  convenient. 

The  observer,  beginning  at  dark,  finds  the  planet 
then  above  the  eastern  horizon.  If,  after  an  interval  of 
three  or  four  weeks,  he  observes  the  planet  at  the  same 
hour,  he  will  find  it,  like  the  constellations  of  fixed  stars, 
situated  farther  from  the  eastern  horizon  and  nearer  the 
western.  If,  at  intervals  of  two  or  three  weeks  during 
several  months,  he  takes  a  look  at  dark,  he  will  again 
and  again  find  the  planet,  like  the  fixed  stars,  farther 
from  the  eastern  horizon  and  nearer  the  western ;  and 
finally  it  would  last  be  seen  at  dark  just  above  the  west- 
ern horizon.  The  observer  would  have  reason  to  think 
that  after  its  disappearance  it  was  on  the  western  horizon 
with  the  sun  at  sunset ;  and,  as  it  was  always  seen  very 
near  the  ecliptic,  it  would  be  evident  that  it  must  at  the 
same  time  be  very  nearly  in  line  with  earth  and  sun. 
This  is  the  planet's  conjunction  with  the  sun. 

If,  after  this,  the  student  made  observation  just  before 
day  in  the  morning,  the  planet  would  be  found  just  above 
the  eastern  horizon.  If,  during  some  months,  it  was  oc- 
casionally observed  at  that  hour,  it  would  gradually  be 
found  farther  and  farther  west,  until  it  would  finally  be 
seen  for  the  last  time  just  above  the  western  horizon. 
Then  it  would  again  be  opposite  the  sun,  or  "  in  opposi- 
tion "  with  him.  It  could  again  be  seen  at  dark  in  the 
evening  just  above  the  eastern  horizon.  Between  the 
two  oppositions  it  would  appear  to  have  made  a  com- 
plete revolution  westward  around  the  earth.  This  revo- 
lution would  have  been  so  like 
the  annual  motion  of  the  fixed 

stars,  that  the  observer  would     EAST I 

suspect  that  it  was  also  appa- 
rent, and  due  to  the  earth's  annual  motion  around  the 
sun. 

But  the  fixed  stars  revolve  round  the  earth  in  a  year, 
crossing  the  evening  or  morning  sky  in  six  months,  while 
Saturn  would  take  a  few  days  more  than  twelve  months 
to  complete  his  revolution  ;  Jupiter  would  take  thirteen 
months  and  Mars  twenty-six. 

126.  Real  or  Proper  Motion. — The  difference  in  the 
times  of  apparent  revolutions  of  the  planets  and  the  fixed 
stars  would  be  explained  by  the  planet's  motion  among 
the  stars,  which  the  observer  would  have  seen  while 
watching  the  synodical  revolution.  It  has  already  been 


noticed  in  123.  The  planets  move  eastward  among  the 
stars.  It  will  perhaps  sound  a  little  absurd  to  speak  of 
a  planet  as  having  two  motions,  one  of  which  is  east- 
ward and  the  other  westward  in  direction.  If  the 
western  motion  were  not  apparent,  it  would  be  absurd. 
The  student  can  form  an  idea  how  the  two  motions  ap- 
pear to  go  on  together,  by  imagining  the  celestial  sphere 
revolving  and  carrying  the  planet  west,  while  the  planet 
at  the  same  time  moves  east  on  the  sphere.  Thus,  a 
globe  might  revolve  in  one  direction,  and  carry  an  ant 
with  it,  while  the  ant  walked  slowly  round  the  globe  in 
the  contrary  direction.  We  have  reason  to  think  the 
eastern  motion  of  a  planet  among  the  stars  a  real  motion, 
for  the  earth  herself  moves  east,  and  so  could  not  make 
the  planet  appear  to  move  east. 

Now  the  planet  would,  at  opposition,  be  at  a  point 
among  the  stars,  and  when  this  point  had  in  six  months 
revolved  to  the  western  horizon,  the  planet  would  be  at 
some  distance  east  of  it,  and  would  thus  be  kept  longer 
above  the  western  horizon.  Since  Mars  moves  east 
among  the  stars  faster  than  Jupiter,  and  Jupiter  than 
Saturn,  Mars  would  be  detained  longest  in  the  evening 
or  morning  sky,  and  Jupiter  would  be  delayed  longer 
than  Saturn. 

The  apparent  western  revolution  of  the  superior 
planets  round  the  earth  is  called  a  Synodical  Revolu- 
tion. This  is  usually  defined  as  a  period  at  the  begin- 
ning and  end  of  which  a  planet  occupies  the  same  position 
in  regard  to  the  earth  and  sun. 

127.  Variation  in  Apparent  Size,  and  Definition  of  a 
Superior  Planet. — If  the  planet  supposed  to  be  watched 
were  Mars,  our  observer  would  see  another  fact  very 
plainly.  Mars  evidently  diminishes  in  apparent  size 
when  crossing  the  evening  sky,  and  increases  while  cross- 
ing the  morning  sky.  Jupiter  and  Saturn  also  vary 
through  the  same  period,  but  the  change  is  not  so  marked. 


FIG.  31. 


-WEST 


Let  us  suppose  that  the  line  above  (see  Fig.  3 1)  repre- 
sents that  drawn  from  east  to  west  through  earth  and  sun 
at  sunset. 

S  is  the  sun's  place ;  E,  the  earth's  place ;  and  e,  the 
point  of  the  earth's  orbit  lying  west  of  the  sun.  At  op- 
position the  planet  was  east  of  the  observer  at  sunset. 
As  he  was  evidently  farther  from  the  sun  then  than  the 
earth  was,  we  mark  his  position  at  P.  For  the  same 
reason  we  put  the  mark  /  for  his  position  in  the  west  at 
conjunction,  beyond  the  sun  and  also  beyond  the  point 
of  the  earth's  orbit.  Now,  if  the  planet  was  at  the  point 
marked  p,  it  explains  why  he  decreased  in  apparent  size. 


ASTRONOMY  BY  OBSERVATION. 


While  P  and  /  are  at  the  same  distance  from  the  sun, 
they  are  at  very  unequal  distances  from  the  earth  at  E. 
When  the  planet  is  at  P,  the  observer  at  E  is  on  the  side 
of  the  earth's  orbit  nearest  him  ;  but  when  he  is  at  /, 
the  observer  at  E  is  on  the  side  of  the  earth's  orbit  most 
distant  from  him.  From  E  to  e  is  a  diameter  of  the 
earth's  orbit,  a  line  more  than  184,000,000  miles  long. 
Therefore,  Mars  appears  larger  when  seen  at  P  than  at 
/,  because  he  is  more  than  184,000,000  miles  nearer. 
Thus  this  increase  of  the  superior  planets  in  apparent 
size  confirms  the  belief  that  they  are  at  all  times  farther 
from  the  sun  than  the  earth  is.  This  is  what  we  mean 
by  a  superior  planet.  It  is  always  at  a  greater  distance 
from  the  sun  than  the  earth  is.  At  opposition  the  planet 
and  earth  are  in  line  with  each  other  and  the  sun,  on  the 
same  side  of  the  sun  ;  at  conjunction,  on  opposite  sides. 

The  student  must  use  this  line  to  get  the  positions  in 
nature,  as  he  can  see  them.  He  should  make  sure  of 
this  by  pointing  to  them.  The  diagram  is  useless  except 
for  this  purpose. 

The  fact  that  Jupiter  varies  less  than  Mars  in  ap- 
parent size,  and  Saturn  less  than  Jupiter,  can  be  accounted 
for  by  supposing  that  the  two  latter  are  so  very  far  from 
earth  and  sun  that  an  approach  of  184,000,000  miles 
nearer  is  comparatively  small. 

128.  The  Sidereal  Revolution. — A  planet  always  lies 
in  a  straight  line  between  the  sun  and  some  star-group, 
since  stars  are  all  round  the  sun ;  but  at  the  opposition 
of  a  planet  we  can  know  the  star-group,  for  we  are  our- 
selves in  line  between  it  and  the  sun.  At  dark  a  line 
from  the  sun,  which  is  just  below  the  western  horizon, 
through  the  earth,  reaches  an  ecliptic  point  just  above 
the  eastern  horizon,  and  at  opposition  the  planet  is  seen 
at  dark  very  near  it.  We  can  see  that  he  is  in  the  same 
direction  from  us  that  we  are  from  the  sun  at  sunset. 
If  we  sit  up  till  midnight  we  see  an  ecliptic  point  on  the 
meridian,  and  the  planet  at  opposition  is  very  near  it. 
The  four  or  five  stars  nearest  the  planet  and  this  point 
are  a  group.  As  we  are  between  this  ecliptic  point  and 
the  sun,  we  are  between  the  sun  and  this  group.  So  is 
the  planet,  though  he  seems  to  be  among  the  group,  but 
we  know  this  is  the  effect  of  projection.  So  we  can  be 
quite  sure  the  planet  is  in  line  between  the  sun  and  this 
group.  If  we  are  not  quite  in  line  with  the  sun  and 
planet,  we  are  between  the  sun  and  the  same  star-group. 

After  a  year's  time  the  same  star-group  would  be 
above  the  eastern  horizon  at  dark,  and  we  should  again 
be  between  the  sun  and  this  group,  but  the  planet  would 
not  be  there.  He  would  have  moved  east  and  would  be 
below  the  horizon.  But  after  a  while  he  would  again 
be  above  the  eastern  horizon  at  dark,  and,  of  course,  op- 
posite the  sun.  He  would  be  between  the  sun  and  a 


second  star-group.  We  should  know  it,  for  we  also 
should  be  between  the  sun  and  the  same  group.  We 
should  see  the  first  and  second  group  both  lying  on  the 
evening  sky,  the  ecliptic  running  through  them,  and  the 
second  group  farthest  east.  Then,  after  a  long  interval, 
the  planet  would  again  be  in  opposition.  We  should 
see  him  between  the  sun  and  a  third  star-group.  The 
three  groups  would  lie  on  the  evening  sky  along  the 
ecliptic,  the  last  one  being  farthest  east.  The  two  in- 
tervals between  the  three  would  be  about  equal.  After 
a  number  of  oppositions,  there  would  be  a  chain  of  these 
groups,  at  equal  intervals,  extending  entirely  across  the 
evening  sky  and  along  the  ecliptic  ;  and  we  should  know 
that  our  planet  had  been  between  the  sun  and  every  one, 
for  we  should  have  been  there  at  the  same  time.  After 
a  still  longer  time,  the  star-groups  would  make  a  ring 
round  the  whole  heavens,  both  above  and  below  the 
horizon.  We  should  know  that  the  planet  had  been 
between  the  sun  and  every  one.  As  they  would  evi- 
dently lie  in  a  ring  all  round  the  sun,  it  would  be  evi- 
dent that  the  planet  had  made  a  complete  revolution 
round  the  sun  (see  Fig.  32). 

Now,  unless  the  annual  motion  of  the  stars  is  real — 


OPPOSITIONS  OF  ^' 


unless  the  earth  is  at  rest,  and  the  sun  and  stars  are  in 
motion — it  is  clear  our  supposed  planet  revolves  around 
the  sun.  This  revolution  of  a  planet  from  one  star-group 


THE  PLANETS  AND    THEIR  MOTIONS,   AND  HOW   TO   OBSERVE   THEM. 


43 


back  to  the  same  is  called  the  planet's  Sidereal  Revolu- 
tion. 

129.  It  would  take  Jupiter  nearly  twelve  years  to 
make  a  sidereal  revolution,  and  it  would  take  Saturn 
nearly  thirty.     But  the  case  is  quite  different  with  Mars. 
His  motion  among  the  stars  is  very  rapid,  and  between 
two  successive  oppositions  he   would  be  seen  moving 
rapidly  east  through  many  zodiacal  constellations.     But 
when  the  second  came,  he  would    be   only  48°  in  ad- 
vance of   his  first  position.     It  is  a  reasonable   conclu- 
sion   that    Mars    makes  a  full  sidereal   revolution,  and 
goes  48°  beyond,  while  he  makes  one  synodical  revolu- 
tion. 

130.  Times  of  Revolutions. — The  student  of  geometry 
knows  that  we  can  not  estimate  circular  or  angular  mo- 
tion except  from   the  center.     But  we  are  not  at  the 
center  of  the  motions  of  the  planets.     Fig.  33,  in  which 
E  represents  the  earth's  position,  and  p,p',p",p",  posi- 
tions of  a  superior  planet,  shows  that  we  could  not  see 

FIG.  33. 


the  planet  where  it  would  be  seen  by  an  observer  at  the 
center,  except  when  we  are  in  line  with  it  and  the  center. 
This  is  the  position  of  sun,  earth,  and  planet  at  conjunc- 
tion and  opposition.  But  we  can  not  see  the  planet  at 
all  at  conjunction.  It  is  this  that  makes  opposition  so 
important  a  period  in  the  study  of  a  planet. 

Now,  when  a  sidereal  revolution  begins  at  an  opposi- 
tion, it  does  not  end  at  one.  We  know  the  planet  has 
completed  its  revolution  because  we  see  it  at  an  oppo- 
sition, in  advance  of  the  point  from  which  it  started. 
Therefore,  we  can  not  estimate  the  length  of  a  sidereal 
revolution  by  counting  days.  But  we  can  count  the 


days  of  a  synodical  revolution,  and  measure  the  angular 
distance  traveled.  Since  the  synodical  revolution  de- 
pends on  the  motion  of  the  earth  and  planet  both,  and 
neither  moves  through  exactly  equal  spaces  in  equal 
times,  both  the  days  and  distance  vary  a  very  little. 
But  we  can  take  the  average,  or  mean,  of  many  obser- 
vations. Then  the  length  of  a  sidereal  revolution  can 
be  found  by  a  question  like  the  following :  If  the  mean 
motion  of  Mars  is  360°+ 48°,  or  408°,  in  779  days,  how 
long  will  it  take  Mars  to  travel  360°  ?  The  answer,  687 
days,  is  the  period  of  the  sidereal  revolution  of  Mars. 

131.  Discussion    of    Appearances.  —  The    apparent 
western  revolution  of  the  planets  is  much   more  con- 
spicuous than  their  real  eastern  motion,  because  in  the 
first   case   they  move  from    east   to    west   of  ourselves. 
But  the  whole  matter  will  be  made  clearer  by  consider- 
ing the  appearances  produced  by  motion  on  earth.     If 
we  walk  past  an  object  at  rest,  it  appears  to  move  in  a 
direction  contrary  to  our  own,  and  at  last  passes  out  of 
sight.    But  let  us  suppose  ourselves  walking  on  a  straight 
road,  and  seeing  in  advance  of  us  a  person  walking  more 
slowly  in  the  same  direction.     He  seems  to  fall  back, 
and  we  catch  up  and  pass  him,  but  he  remains  in  sight 
longer  than  the  object  at  rest.     If  he  quickens  his  pace, 
still,  however,  walking  more  slowly  than  we  do,  he  still 
appears  to  fall  back,  but  more  slowly,  and  he  keeps 
longer  in  sight  of  us. 

132.  The  fixed  stars  are  objects  at  rest,  and  Mars, 
Saturn,  and  Jupiter  are  like  persons  walking  in  the  same 
direction  with  ourselves,  but  at  different  degrees  of  speed, 
round  a  circle.     When  a  planet  and  the  earth  come  in 
line  at  opposition,  it  is  because  the  earth,  moving  east 
faster,  catches  up  with  the  planet  and  passes  him.    When 
they  come  in  line  at  conjunction,  it  is  because  the  earth 
has  traveled   180°  ahead  of  the  planet,  and  is  coming 
round  the  circle  behind  him,  to  catch  up  and  pass  him 
again.     But  the  earth's  motion  is  without  jolt,  jar,  noise, 
or  exertion  on  our  part,  and  we  do  not  feel  that  we  are 
moving,  so  we  take  the  appearance  of  the  planet  falling 
back  to  be  the  real  motion. 

133.  Apparent  Retrograde  Motion. — This  is  a  very 
important  apparent  movement ;  but  the  description  of  it 
has  been  postponed  in  order  to  avoid  confusion.     Thus 
far  all  the  facts  and  appearances  can  be  explained  almost 
equally  well  on  the  supposition  that  the  earth  is  at  rest, 
while  sun,  planets,  and  stars  move  round  it  with  vary- 
ing degrees  of  speed,  as  on  the  supposition  that  the  sun 
is  the  center  of  motion  around  which  earth  and  planets 
revolve.     But  it  is  otherwise  with  the  interesting  move- 
ment about  to  be  described.     It  can  be  better  explained 
on  the  theory  of  the  earth's  motion.     This  movement 
had  great  influence  in  making  astronomers  believe  the 


44 


ASTRONOMY  BY  OBSERVATION. 


Copernican  theory,  by  which  the  sun  is  supposed  to  be 
the  center,  and  the  planets  to  revolve  round  him. 

134.  For  a  few  weeks  before  and  after  the  opposition 
of  a  superior  planet,  it  seems  to  move  west  among  the 
stars  instead  of  east.     If  we  supposed  that  this  motion 
was  real,  not  apparent,  we  should  see,  by  the  advance 
east  among  the  stars  at  opposition,  that  the  planets  must 
travel  longer  and  farther  east  than  west.    But  the  move- 
ment is  apparent,  not  real. 

135.  The  student  will  best  understand   the  cause  of 
this  apparent  western  or  retrograde  motion  by  an  ex- 
periment.    On  ground,  level  and  open,  for  fifty  feet  ra- 
dius, he  must  draw  two  concentric  circles  with  radii  of 
about  twelve  and  eighteen  feet.     He  is  to  have  an  as- 
sistant who  walks  slowly  around  the  outer  circle,  while 
he  himself  walks  around  the  interior  circle,  a  little  fast- 
er, but  in  the  same  direction.     At  intervals  he  comes 
up  in  line  with  his  assistant  and  the  center,  on  the  same 
side  of  the  center  (or  on  the  same  radius),  and  passes  his 
assistant.     While  walking  on  all  parts  of  the  circle,  he 
notes  the  passage  of  his  assistant's  head  over  the  back- 
ground.    The  apparent  and  real  directions  of  the  walk- 
er's motion  on  the  outer  circle  are  the  same,  until  just  be- 
fore the  two  come  in  line  (on  the  same  radius),  and  then 
the  observer  sees  his  assistant's  head  retrograde  over  the 
background,  or  move  in  a  direction  contrary  to  that  in 
which  the  observer  knows  he  is  really  moving.     The 
position  of  the  three,  when  in  line,  is  that  of  S,  E,  and 
p,  in  Fig.  33.     This  is  just  the  position  of  the  earth  and 

a  superior  planet  at 

FlG-  34~  opposition.      They 

are  in  line  on  the 
same  side  of  the 
sun. 

There  is  noth- 
ing mysterious  in 
this.  When  two 
walkers  on  straight 
paths  walk  with 
different  degrees 
of  speed  in  the 
same  direction,  the 
one  in  advance 
walking  more  slow- 
ly, and  the  one  in 
the  rear  catching 
up  and  passing  the  other,  the  fast  walker  will  see  the 
body  of  the  slow  walker  move  over  a  distant  enough 
background  in  a  direction  contrary  to  that  in  which  he 
really  walks.  In  the  circles  of  Fig.  34,  the  arrows  indi- 
cate the  movement  of  revolution,  and  it  can  be  seen  that 
a  mover  at  and  near  a  is  not  moving  in  the  same  abso- 


lute direction  with  a  mover  on  the  outer  circle,  except 
when  he  is  at  or  near  b.  The  reason  why  the  motion  of 
the  planet  retrogrades  near  opposition  only,  is,  the  earth 
and  planet  are  not  moving  in  the  same  direction  except 
at  and  near  that  time. 

136.  There  is  another  point  to  be  noticed.     Jupiter 
retrogrades  through  a  smaller   angular   distance   than 
Mars,  and  Saturn  than  Jupiter.     If  the  experiment  with 
the  circles  is  tried  as  before,  except  that  the  outer  cir- 
cle has  a  radius  of  twenty-five  or  thirty  feet,  it  will  be 
found  that  the  walker  on  the  outer  circle  retrograde? 
through  a  smaller  angular  distance  than  before. 

137.  To   explain   this   retrograde    motion,    we    must 
make   one   of   two   suppositions:    i.  The   earth    moves 
round  the  sun  in  a  figure  nearly  a  circle,  and,  catching 
up  in  line  with  the  superior  planets,  makes  them  seem  to 
move  backward,  like  the  walker  in  the  experiment  with 
the  circles.     2.  The  earth  is  really  at  rest,  and  the  plan- 
ets are  in  motion,  but  the  planets,  for  no  reason  at  all  that 
we  can  conceive,  imitate  the  movements  they  would  ap- 
pear to  have  if  the  earth  moved. 

It  is  clear  that  the  first  is  the  more  reasonable  suppo- 
sition. 

138.  Several  conclusions   follow  :    i.  This  motion  of 
the  planets  toward  the  west  is  apparent.     2.  Since  the 
earth's  motion  gives  the  planets   an   apparent   motion 
among  the  fixed  stars,  while  the  fixed  stars  themselves 
are  too  far  away  to  change   place  among   each   other 
in  consequence  of  the  earth's  motion,  the  planets  must 
be  a  great  deal   nearer  to  us  than  the  fixed  stars.     3. 
Mars  is  nearer   to   us   than   Jupiter,  and  Jupiter   than 
Saturn. 

139.  Summary  of  what  the  Student  should  observe. 
—The  student  will  probably  think  that,  if  he  must  watch 

Jupiter  twelve  years  and  Saturn  thirty,  to  become  satis- 
fied that  they  revolve  round  the  sun,  he  will  have  to  be 
contented  with  a  blind  acceptance  of  the  statements  of 
school-books. 

In  astronomy,  as  on  other  subjects,  we  accept  the 
evidence  of  witnesses  to  fact.  But  on  this,  as  on  all  sci- 
entific subjects,  there  is  a  great  difference  between  a 
blind  and  an  intelligent  acceptance  of  testimony.  We 
must  know  something  of  the  nature  of  the  facts,  or  we 
can  not  tell  whether  they  are  rightly  interpreted.  The 
motions  here  described  consist  largely  of  repetitions. 
But  we  can  understand  them  perfectly,  long  before  we 
go  through  with  all  the  repetitions. 

But  the  student  who  learns  to  look  at  the  heavens 
with  intelligence,  will  see  a  great  deal  more  of  the  repe- 
titions than  he  intends  at  first.  The  heavens  are  unrolled 
before  us  year  by  year,  without  any  exertion  on  our 
part,  and  just  at  the  time  when  we  are  most  at  leisure. 


THE  PLANETS  AND    THEIR  MOTIONS,   AND  HOW   TO   OBSERVE   THEM. 


45 


We  must  shut  our  eyes,  or  look  down,  to  keep  from  see- 
ing them.  Thus  the  person  who  learns  to  understand 
the  changes  in  the  heavens  is  apt  to  become  an  observer 
for  life. 

The  following  phenomena  should  be  observed:  I. 
The  positions  of  one  or  two  of  the  planets  in  regard  to 
earth  and  sun  should  be  noted  near  conjunction  and 
at  opposition,  and  thoroughly  understood.  2.  The  de- 
crease in  brilliancy  from  opposition  to  conjunction  should 
be  noted.  This  is  best  seen  in  the  case  of  Mars.  3. 
The  general  eastward  motion  of  all  these  three  planets 
should  be  noticed  ;  also,  the  western  or  retrograde  mo- 
tion among  the  stars  near  opposition  should  be  noted. 

4.  The   positions   among  the   stars  for   two   successive 
oppositions  should  be  noted,  in  the  case  of  Jupiter  and 
Saturn  especially.     This  shows  the  advance  eastward. 

5.  The  apparent  motion   toward   the    western   horizon 
should  be  noted,  and  especially  the  period  during  which 
each  planet  is  in  the  evening  sky.     6.  Besides  this,  the 
positions  of  the  planets  in  regard  to  the  ecliptic  must 
be   noted.      But   the   student   must  clearly  understand 
how  much,  how  little,  this  proves.     It  shows  with  entire 
truth  that  the  orbits  of  these  planets  are  in  planes  very 
near  the  ecliptic.     But  it  docs  not  show  the  exact  planes 
in  which  they  move.     As  we  are  sometimes  a  little  to  one 
side  of   the   planes  in  which   they  move,  sometimes  a 
little  to  the  other  side,  we  see  them  a  very  little  dis- 
placed.    But    it   is    perfectly  correct  to  say  that   they 
are    always   so    near   the   ecliptic    because   they   move 
in  planes  differing  so  little  from  that  in  which  the  earth 
moves.     In  no  way  can  the  student  understand  this  so 
well  as  by   watching   their  position  in  regard   to   the 
ecliptic. 

The  Inferior  Planets. 

140.  Definition   of  an   Inferior   Planet. — Venus  and 
Mercury  are  never  seen  near  the  point  of  the  heavens 
opposite  the  sun.     If  they  were  not  nearer  the  sun  than 
the  earth  is,  the  earth  would  certainly  come  between 
them  and  the  sun  at  some  point  of  her  revolution  round 
that  center.    But  we  always  see  them  not  far  from  where 
we  know  the  sun  is  situated.     For  this  reason,  and  for 
others  which   will  appear  in  studying  them,  they  are 
supposed  to  move  round  the  sun  in  orbits  interior  to  the 
earth's  orbit,  and  astronomers  call  them  the  "  Inferior 
Planets."      The  student's   knowledge  of   their  motions 
must  be  chiefly  gained  from  the  study  of  Venus.    Venus 
is  very  often  so  situated  that  we  can  observe  her  with 
the  greatest  convenience. 

141.  When  and  how  to  find  Venus. — Venus  is  alter- 
nately evening  and  morning  star  for  periods  of  292  days 
each.     She  becomes  evening  star  at  what  is  called  her 


superior  *  conjunction  with  the  sun,  and  she  becomes 
morning  star  at  her  inferior  *  conjunction  with  the  sun. 
The  symbol  of  Venus  is  ?  ;  that  of  conjunction,  9  ;  that 
of  the  sun,  O.  The  entries  in  the  almanac  are  "  p  $  O 
superior"  and  "p  ?Q  inferior."  They  are  found  at 
the  proper  dates.  Besides  these  announcements,  most 
almanacs  have,  in  the  beginning,  another,  giving  the 
evening  and  morning  stars  for  the  year. 

When  Venus  is  known  to  be  above  the  horizon  in  the 
evening,  it  is  not  possible  to  fail  in  identifying  her,  for 
she  is  found  at  dark  in  the  west,  and  is  far  the  brightest 
star  visible.  But  the  student  will  not  be  able  to  see  her 
for  some  time  after  conjunction.  The  delay  depends  on 
the  angle  made  by  the  path  of  Venus  with  the  horizon, 
and  on  the  clearness  of  the  weather.  She  should  be 
looked  for  three  weeks  after  conjunction,  and  after  that 
once  a  week,  at  least,  until  she  is  found. 

If  the  student  prosecutes  diligent  search,  she  will  be 
seen  at  first  so  early  after  sunset  that  no  other  stars  will 
then  be  visible.  The  observer  should  note  and  remem- 
ber the  point  of  the  horizon  above  which  he  first  sees  her. 

142.  Diurnal    Revolution.  —  On    the    evening    when 
Venus  is  first  seen  she  will  set  in  the  west  just  as  the 
new  moon  does.     As  all  the  other  heavenly  bodies  do 
this,  it  will  be  plain  that  the  motion  is  apparent  and  due 
to  the  earth's  axial  rotation,     On  the  next  evening  she 
will  again  be  seen  in  the  west,  and  it  will  be  evident  that 
she  must  have  risen  on  the  same  morning  a  little  later 
than  the  sun,  and   during  the  day  revolved  (invisible) 
across  the  sky,  keeping  near  to  the  sun  on  his  eastern 
side,  and  becoming  visible  when  he  has  set. 

143.  Real  Motion. — After  a  few  weeks  the  observer 
would  see  that,  at  the  same  hour  of  the  evening  at  which 
Venus  was  first  observed,  she  was   higher  above   the 
horizon  and  farther  east.     It  would  be  clear  that  this 
was  due  to  a  motion  of  the  planet,  real  or  apparent.     As 
the  earth  herself  moves,  not  west,  but  east,  she  could  not 
make  the  planet  appear  to  move  east.     Thus  it  would 
be  plain  that  the  motion  of  Venus  from  the  horizon  was 
her  real  or  proper  motion.     From  this  it  would  also  fol- 
low that  she  must  have  come  into  the  evening  sky  by 
rising  above  the  western  horizon,  and  must  have  caught 
up  with  and  passed  the  sun.     It  would  be  plain  that  her 
real  motion   was  faster  than  the   sun's   eastern  motion, 
and  that  she  must  therefore  move  faster  than  the  earth. 
When  she  thus  passes  the  sun,  she  is  at  conjunction. 

After  a  long  time,  Venus  would  be  far  enough  east 
at  sunset  to  be  visible  when  the  stars  were  seen,  and  she 
would  be  found  moving  east  among  the  stars.  Just  as 
soon  as  he  could,  the  student  should  fix  her  place  accu- 

*  This  use  of  the  words  superior  and  inferior  will  be  understood  in  study- 
ing the  planets'  motions. 


ASTRONOMY  BY  OBSERVATION. 


rately  in  relation  to  the  fixed  stars,  so  that  he  could  de- 
tect her  motion  among  them.  She  moves  so  rapidly 
that  her  changed  position  would  be  evident  the  very 
next  evening.  This  rapid  motion  makes  it  very  interest- 
ing work  to  trace  the  path  of  Venus  among  the  stars. 

144.  Venus  and  the  Ecliptic. — One  point  which  should 
be  noticed  in  watching  her  is  the  position  of  her  path  in 
relation  to  the  ecliptic.     She  keeps  very  near  it,  and  the 
student  should  note  whether  she  is  north  or  south  of  it. 
But  he  must  constantly  remember  what  was  said  in  139. 
The  unvarying  nearness  of  the  planets  to  the  ecliptic 
shows  that  they  move  in  planes  very  near  it.     But  it 
does  not  show  the  exact  planes  of  their  orbits. 

145.  Elongations,  etc. — The  observer,  watching  Venus, 
would  see  her  continue  to  get  higher  above  the  horizon, 
and  farther  east  among  the  stars.     The  earth's  motion 
in  her  orbit  would  cause  many  constellations  in  which 
Venus  had  been  seen  to  go  out  of  sight  behind  the  west- 
ern horizon.     Finally,  219  days  after  superior  conjunc- 
tion, the  almanac  would  contain  the  notice  "  ?    gr.  El. 
E.,"  or  the  greatest  Eastern  Elongation  of  Venus.     That 
is,  Venus  would  have  reached  her  greatest  height  above 
the  western  horizon,  and  would  afterward  begin  to  ap- 
proach it.     Venus  would  be  nearly  half-way  between 
the  zenith  and  the  horizon  at  the  earliest  hour  at  which 
she  could  be  seen  on  the  evening  of  her  greatest  elonga- 
tion.    Lines  drawn  from  the  observer's  eye  to  Venus 
and  the  sun  would  make  an  angle  of  about  47°.     The 
angle  varies  a  very  little  at  different  elongations,  but  the 
almanacs  usually  give  the  exact  number  of  degrees.     It 
would  be  a  good  many  days  before  the  ordinary  ob- 
server without  instruments  would  perceive  that  Venus 
was  getting  nearer  the  horizon.     After  a  time  it  would 
be  very  plain.     But  Venus  would  continue  to  move  east 
among  the  stars  until  about  270  days  after  superior  con- 
junction, when  she  would  begin  to  move  west  among  the 
stars,  and  would  continue  to  do  so  while  she  remained 
in  the  evening  sky.     Finally,  292  days  after  Venus  be- 
came evening  star,  the  almanac  would  announce,  "  p  ?  O 
inferior,"  and  Venus  would  pass  the  sun  again  as  she 
went  down.     But  the  observer  would  know  the  exact 
time  by  the  almanac  only,  for  he  would  see  the  last  of 
Venus  some  days  earlier.     The  observer  should  note 
the  point  of  the  horizon  above  which  Venus  was  last 
seen.     It  would  perhaps  be  a  little  north  or  south  of  the 
point  above  which  she  was  first  seen.     Thinking  of  her 
motion  in  relation  to  the  horizon  only,  and  forgetting 
the  movement  among  the  stars,  it  would  seem  to  the  ob- 
server that  the  figure  of  her  motion  was  a  long  half  oval 
with  its  base  resting  on  the  horizon. 

146.  Increased  Brilliancy. — It  would  be  very  evident 
to  a  person  who  watched  Venus   closely,  that  she  in- 


creased in  brilliancy  from  the  time  she  became  evening 
star.  She  would  be  a  splendid  object  when  she  left  the 
evening  sj^y.  These  facts  would  indicate  that  she  was 
getting  nearer  to  the  earth  all  the  time  that  she  was 
evening  star. 

147.  Explanation,  or   Theory. — If   the   student   will 
cut  a  circle  of  about  six  inches  in  diameter  from  rather 
stiff  paper,  he  will  see  that  he  can  hold  it  west  of  him  in 
such  a  position  that  it  will  look  like  a  long  oval,  or  even 
like  a  line.     When  held  in  this  position,  a  body  supposed 
to  move  around  its  circumference  would  be  much  nearer 
to  the  observer  when  on  one  side  of  it  than  when  on  the 
other.     If  the  body  moved  in  the  direction  in  which  the 
earth  revolves  round  the  sun,  it  would  be  at  the  greatest 
distance  from  the  observer  when  it  was  going  up,  and 
nearest  him  when  moving  down.    Now  the  earth's  orbit, 
which  is  always  in  the  direction  of  the  sun,  is  always 
west  of  us  at  sunset,  and  only  half  of  it  is  above  the 
horizon.     If  Venus  revolves  around  the  sun  in  an  orbit 
interior  to  the  earth's  orbit,  it  might  be  situated  in  such 
a  plane  as  to  look  like  a  long  oval,  of  which  half  was 
above  the  horizon — or  even  like  a  line.     If  Venus  moves 
around  it  in  the  same  direction  in  which  the  earth  re- 
volves round  the  sun,  she  would  seem  to  move  in  such 
a  long  semi-oval,  first  up,  then  down ;   and   when  she 
passed  the  horizon  moving  up,  she  would  be  much  far- 
ther from    the  earth   and  observer  than  when  coming 
down.     Thus  this  theory  would  account  for  her  rising 
47°  and  then  coming  down.     And,  as  she  would  be  get- 
ting nearer  to   the  earth  from  the  time  that   she  rose 
above  the   horizon,  her  increased    brilliancy  would  be 
accounted  for.     The  fact  that  Venus  is  never  seen  oppo- 
site the  sun  would  also  be  explained.     It  is  because  we 
never  come  between  her  and  the  sun. 

148.  Superior    and    Inferior  Conjunction. — If  Venus 
moves  in  an  orbit  interior  to  the  earth's  orbit,  and  in  the 
same  direction  in  which  the  earth  moves  round  the  sun, 
she  must  be  farther  west  than  the  sun  is  when  she  passes 
him  coming  up  into  the  evening  sky.     And,  also,  when 
she  passes  him  moving  down  out  of  the  evening  sky, 
she  must  be  nearer  to   the  earth  than  the  sun  is.     As 
Venus  is  always  very  near  the  ecliptic,  she  must  be  very 
nearly  in  line  with  sun  and  earth  in  both  cases.     At  su- 
perior conjunction,  an  inferior  planet,  the  sun  and  earth 
are  nearly  in  line,  with  the  sun  in  the  middle.     At  infe- 
rior conjunction,  they  are  nearly  in  line,  with  the  planet 
in  the  middle. 

149.  Phases   of  Venus. — A  telescope   reveals    some 
facts  which  strongly  confirm  this  explanation  of  the  mo- 
tions of  Venus.     If,  at  superior  conjunction,  the  earth 
and  Venus  are  on  opposite  sides  of  the  sun,  she  has  the 
same  face  turned  to  the  earth  and  sun.     This  is  exactly 


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THE  PLANETS  AND    THEIR  MOTIONS,   AND  HOW   TO   OBSERVE    THEM. 


47 


the  position  of  the  moon  when  she  is  full.  Also,  just 
after  inferior  conjunction,  if  she  is  then  nearer  to  us  than 
the  sun  is,  she  is  in  exactly  the  position  of  the  new  moon. 
Also,  at  her  elongation,  we  see  half  of  the  side  turned  to 
the  sun,  just  as  we  see  the  moon  at  her  quadrature. 
Now,  when  Venus  is  observed  through  the  telescope, 
she  exhibits  phases  just  as  the  moon  does,  except  that 
she  enters  the  evening  sky  a  full  circle  and  leaves  it  a 
crescent.  But  these  changes  proceed  concurrently  with 
the  increase  in  apparent  size.  The  full  circle  is  much 
smaller  than  that  of  which  the  crescent  forms  part.  Fig. 

FIG.  35. 


35  shows  the  relative  proportions  of  Venus  at  her  supe- 
rior and  inferior  conjunctions. 

About  a  month  before  the  inferior  conjunction, 
there  is  an  entry  in  the  almanac,  "  $  at  greatest  brill- 
iancy." The  brilliancy  of  Venus  depends  on  two 
things,  her  apparent  size  and  the  amount  of  illuminated 
surface.  At  the  date  indicated,  the  combined  effect 
of  the  two  is  greatest.  The  phases  of  Venus  show 
that,  like  the  moon,  she  shines  by  reflecting  the  light  of 
the  sun.* 

150.  Transit  of  Venus. — The  argument  to  show  that 
Venus  moves  round  the  sun  in  an  orbit  interior  to  the 
earth's  orbit  is  very  strong  without  the  aid  of  the  facts 
learned  from  the  telescope.     It  is  irresistible  with  them ; 
but  it  is  still  further  strengthened.    If  Venus  is  crossing 
the  ecliptic  at  her  inferior  conjunction,  she  is  seen  to 
pass  across  the  face  of  the  sun  like  a  small  black  ball. 
This  is  called  a  Transit  of  Venus,  and   it  never  takes 
place  at  superior  conjunction.     Two   transits   only  oc- 
cur in  a  century,  with   an   interval  of   eight  years  be- 
tween them.     The  last  two  transits  occurred  in  1874  and 
1882. 

151.  Venus  as  Morning  Star. — After  her  inferior  con- 
junction, we  must  look  for  Venus  before  sunrise  in  the 
morning  sky.     Her  motions  are  the  same  as  when  she 

*  Mars  shows  a  little  indication  of  phases.  We  do  not,  in  certain  posi- 
tions, see  him  a  perfect  circle. 


is   evening   star,    except   that    they    occur  in   inverted 
order.     The  almanac  records  them  in  order. 

152.  Western  or  Retrograde  Motion. — The  western 
or  retrograde  motion  of  Venus  has  not  the  importance 
or  interest  of   the  retrograde    motion  of   the  superior 
planets.     It  is  not   an  apparent,  or    parallactic  motion 
caused  by  our  motion.     If  the  student  will  take  some 
small  object,  and  revolve  it  in  a  circle  between  himself 
and  the  wall,  he  will  see  that  in  the  parts  of  the  circle 
nearest  him  and  farthest  from  him,  it  moves  in  opposite 
directions  against  the  background  of  the  wall.     Thus, 
Venus  really  moves  up  and  down.    We  see  both  motions 
against  the  same  background.     One  is  in  the  order  of 
the  signs — Aries,  Taurus,  Gemini,  etc. — and  we  are  ac- 
customed to  call  this  eastward  and  direct,  so  we  call  the 
other  western  and  retrograde. 

153.  Synodical  Period. — Venus  does  not,  like  the  su- 
perior planets,  make  an  apparent  revolution  round  the 
earth.    But  the  period  at  the  beginning  and  end  of  which 
she  occupies  the  same  position  in  regard  to  the  sun  and 
earth   is  called  her  synodical  period.     She  does  this  at 
successive  superior  or  inferior  conjunctions,  but  she  is  too 
near  the  sun  for  us  to  see  her.     At  her  elongations,  lines 
drawn  between   the  sun,  the  earth,  and  Venus  form  a 
right  triangle.     The  reader  will  see  that  this  must  be  so, 
by  examining  Fig.  36.     When  the  planet  in  the  figure  is 
seenatitsgreat- 

est  altitude,  the 
line  to  the  earth 
is  a  tangent  to 
its  orbit,  and 
makes  a  right 
angle  with  the 
line  from  the 
planet  to  the 
sun.  But  Venus  is  at  her  greatest  height  above  the 
horizon  at  her  elongations.  So  we  can  count  her  synodic 
period  in  days,  by  the  time  from  her  eastern  or  western 
elongation  back  to  the  same.  The  average  time  is  584 
days.  This  is  also  the  number  of  days  between  her  su- 
perior conjunctions.  She  is  evening  star  and  morning 
star  292  days  each  (on  an  average). 

154.  Sidereal  Revolution. — We  learn  the  period  of  the 
synodical  revolution,  as  given  above.     Circular  motion 
is  measured  from  the  center ;  but  we  are  not  only  not 
at  the  center  of  the  orbit  of  Venus,  we  are  wholly  out 
of  it.     But  at  her  conjunctions,  we  can  tell  where  she 
would  be  seen  by  an  observer  at  the  sun.     At  superior 
conjunction,  when  she  is  on  the  western  side  of  the  sun 
at  sunset,  an  observer  at  the  sun  would  see  her  where 
we  do,  on  the  western  horizon.     But  at  inferior  con- 
junction, when  she  is  between  us  and  the  sun  at  sunset, 


FIG.  36. 


ASTRONOMY  BY  OBSERVATION. 


an  observer  at  the  sun  would  see  Venus  just  180°  from 
where  we  do,  viz.,  on  the  eastern  horizon.  Therefore,  but 
for  the  motion  of  the  constellations,  we  would  say  Venus 
had  traveled  180°  while  evening  star.  But  it  is  evi- 
dent that  Venus  passes  through  every  constellation  that 
passes  the  sun  while  she  is  evening  star.  That  is,  Venus, 
while  evening  star,  travels  as  far  as  the  sun  and  180° 


besides.    The  sun's  mean  daily  journey  is 


360° 


-,  or  .986°, 


365* 

and  his  mean  journey  in  292  days  is  288°.  Therefore  the 
mean  distance  traveled  by  Venus  in  292  days  is  180°  + 
288°,  or  468°.  Her  mean  distance  in  one  day  is  1.6°,  and 
she  would  travel  360°  in  225  days.  This  is  the  period 
of  the  sidereal  revolution  of  Venus. 

155.  Mercury. — The  synodical  period  of  Mercury 
occupies  1 16  days,  though  his  sidereal  revolution  takes 
only  about  88  days  ;  therefore,  he  is  evening  star  every 
alternate  58  days.  This  event  is  recorded  in  the  alma- 
nac by  the  symbols  "  6  9  O  sup."  But  though  he  comes 
so  often,  there  are  a  good  many  difficulties  in  getting  a 
sight  of  him.  He  is  never  more  than  28°  from  the  sun 
at  his  greatest  elongation.  His  orbit  differs  from  a  cir- 
'cle  more  than  that  of  any  other  planet ;  *  and,  when  he  is 
at  perihelion  and  elongation  at  the  same  time,  he  is 
much  nearer  the  sun.  Besides,  he  gets  farther  from  the 
ecliptic  than  any  other  planet.*  The  most  favorable 
times  for  seeing  him  are  when  he  is  evening  star  in  the 
spring,  and  morning  star  in  the  faJJ.  He  is  very  bright 
when  he  is  in  the  field  of  vision,  and  can  not  be  mistaken. 
He,  is  of  course,  to  be  looked  for  near  the  horizon  and 
ecliptic.  By  watching  the  almanac  in  spring  and  fall, 
for  his  two  conjunctions,  the  industrious  student  will  be 
sure  to  see  Mercury  in  the  end. 

The  motions  of  Mercury  are  in  all  respects  similar  to 
those  of  Venus.  He  moves  more  rapidly.  He  does  not 
vary  in  apparent  diameter,  nearly  so  much  as  Venus,  be- 

FIG.  37. 


Phases  of  Mercury,  and  its  Comparative  Size  as  seen  at  Different  Times. 

cause  the  diameter  of  his  orbit,  which  measures  the  vari- 
ation of  his  distance  from  us,  is  much  less  than  that  of 
Venus.  (See  Fig.  37.) 

0  Except  the  asteroids,  to  be  hereafter  mentioned. 


CHAPTER    VII. 

THE    ATTRACTION    OF   GRAVITATION. 

156.  After  learning  something  of  the  motions  of  the 
planets,  it  is  natural  that  we  should  desire  to  know  the 
force  which  causes  them  to  move  in  their  orbits. 

The  mere  fact  that  the  planets  move,  does  not  need 
to  be  accounted  for.  If  we  roll  a  ball  on  a  slightly  rough 
surface,  it  continues  to  move  after  the  hand  is  removed, 
but  finally  stops.  In  proportion  as  we  lessen  friction 
and  the  resistance  of  the  air,  its  motion  lasts  longer. 
Therefore,  it  is  believed  that  a  body  moving  where  there 
is  no  friction  and  no  resistance  of  the  air  would  keep  on 
moving  forever.  The  planets  are  placed  where  there  is 
no  friction  and  no  resistance  from  the  air,  for  they  carry 
their  atmospheres  with  them.  The  continued  motion  of 
the  planets  is  regarded  as  a  final  proof  of  the  law  of  iner- 
tia, viz.,  a  body  at  rest  will  continue  at  rest  until  some 
force  sets  it  in  motion,  and  a  body  in  motion  will  con- 
tinue to  move  until  some  force  stops  it. 

But  on  earth  bodies  in  motion  always  move  in  straight 
lines  unless  some  force  deflects  their  motion.  If  they 
move  in  a  curve,  they  change  direction  continually  ;  and 
therefore  the  force  must  be  constant,  not  instantaneous, 
in  operation. 

It  has  always  been  known  that  some  force  draws 
bodies  toward  the  center  of  the  earth,  and  that  they 
move  in  straight  lines  unless  some  other  force  acts  on  a 
falling  body,  when  it  moves  in  a  curve.  Thus  we  may 
throw  a  stone  horizontally,  but,  since  the  attraction  of  the 
earth  affects  it,  it  falls  in  a  curve.  If  we  revolve  a  key 
fastened  to  the  end  of  a  string,  we  project  it  in  a  straight 
line,  as  is  shown  as  soon  as  we  let  go  the  string.  The 
force  of  cohesion  in  the  string,  holding  it  to  the  center, 
makes  it  move  in  a  curve. 

We  have  proof  that  the  earth's  attraction  may  be 
modified,  for,  when  we  revolve  a  bucket  of  water  rapidly, 
the  water  does  not  fall  out.  So  we  may  surmise  that 
the  moon  is  kept  from  falling  to  the  earth  by  her  rapid 
motion,  and  that  the  earth's  attraction  makes  the  motion 
a  curve. 

157.  Now  the  great  mathematician,  Sir  Isaac  Newton, 
proved  that  the  attraction  of  matter  is  the  force  which 
makes  the  heavenly  bodies  move  as  they  do ;  but  his  ar- 
guments were  nothing  like  these,  though   he  took  for 
granted  the  known  laws  of  motion  in  his  reasoning. 

158.  Before   the   time   of   Newton,   astronomers  ob- 
served the  heavenly  bodies  as  we  have  described  in  the 
previous  chapters,  except  that  they  had  instruments  for 
measuring  angles,  and  they  subjected  everything  to  exact 
measurement.     They  measured  the  apparent  diameters 
of  sun  and  moon,  the  angles  which  inferior  planets  make 


THE  ATTRACTION  OF  GRAVITATION. 


49 


FIG.  38. 


with  the  sun  at  their  elongations,  they  measured  the 
angles  through  which  the  superior  planets  retrograde, 
and  a  great  many  other  things.  Since  Jupiter  and  Sat- 
urn revolve  very  slowly,  and  since  it  takes  much  repe- 
tition of  observation  to  insure  accuracy  of. result,  the 
reader  can  see  how  much  slow,  patient,  unobtrusive 
work  somebody  did  before  anything  definite  could  be 
known.  An  astronomer  called  Tycho  Brahe  collected 
a  vast  mass  of  accurate  information.  He  was  fortunate 
enough  to  have  some  property,  and  to  find  a  munificent 
King  of  Denmark,  who  built  an  observatory  called 
Uranienburg,  on  an  island  for  him,  and  gave  him  a  com- 
fortable salary,  so  that  he  could  watch  and  measure  the 
heavens  in  peace  and  quiet.  He  kept  at  it  for  more  than 
twenty  years. 

159.  Then  an  astronomer  called  John  Kepler  took 
Tycho's  measurements,  and,  after  a  great  deal  of  hard 
trying  and  thinking,  came  to  certain  very  definite  results 
about  the  times,  distances,  and  orbits  of  the  planets.  They 
are  called  Kepler's  Three  Laws,  and  they  are .given  below : 

1.  All  the  planets  move  from  east  to  west  in  ellipses 
which  have  the  sun  for  a  common  focus. 

2.  The   radius   vector   of   a   planet    describes   equal 
areas  in  equal  times.     (The  radius  vector  is  the  moving 
line  from  the  planet  to  the  sun.)     This  is  illustrated  in 

Fig.  38.  In  order 
that  this  may  be 
true,  the  planets 
must  move  fastest 
when  near  the  sun. 
3.  The  squares 
of  the  periodic 
fc  times  (sidereal  rev- 
olutions) of  the 
planets  are  to  each 
other  as  the  cubes 
of  their  mean  dis- 
tances from  the 
sun. 

Now,  philosophers  did  not  consider  it  proved  that  the 
attraction  of  matter  causes  the  planets  to  move  in  curved 
lines  until  the  result  of  the  motion  had  been  accurately 
measured,  and  the  force  estimated  in  numbers,  and  shown 
to  be  mathematically  equal  to  producing  the  result.  To 
reason  otherwise  would  be  as  absurd  as  to  pronounce 
a  piece  of  carpeting  exactly  sufficient  to  cover  a  floor, 
without  measuring  the  floor  and  the  carpeting.  When, 
by  the  help  of  Tycho  Brahe  and  Kepler,  the  times,  the 
distances  from  the  sun,  the  orbits,  were  accurately 
measured,  then,  and  not  before,  it  was  time  for  some- 
body equal  to  the  task  to  try  to  make  a  mathematical 
estimate  of  the  force. 
7 


Illustration  of  Kepler's  Second  Law. 


160.  Sir    Isaac  Newton  then  took  up   the   problem, 
and  by  a  long  chain  of  difficult  calculation  belonging  to 
the  higher  mathematics  showed  that  all  the  facts  would 
be  accounted  for  by  the  following  law : 

"  Every  particle  of  matter  in  the  universe  attracts 
every  other  particle  with  a  force  proportioned  directly 
to  the  mass  (or  quantity  of  matter)  and  inversely  to  the 
square  of  the  distance  between  them." 

When  we  consider  that,  besides  the  sun,  there  are 
eight  large  planets  with  their  moons,  of  various  masses, 
and  at  various  distances  from  the  sun,  we  see  what  a 
complicated  piece  of  calculation  it  is  to  show  that  this 
law  accounts  for  the  deflection  of  their  motions  from 
straight  lines.  Sir  Isaac  Newton  did  not  finish  this  prob- 
lem in  its  details,  but  he  proved  enough  to  make  mathe- 
maticians quite  confident  that  his  solution  was  correct. 
There  were  certain  irregularities  in  the  motions  which 
were  not  fully  accounted  for.  These  are  called  pertur- 
bations. Succeeding  astronomers  have  largely  supplied 
the  details — notably  two  Frenchmen,  Laplace  and  La- 
grange — and  fresh  facts,  learned  by  observation,  have 
aided  in  this  work;  but  there  is  still  something  to  be 
done. 

161.  In  1846  there  was  a  curious  example  of  the  way 
in  which  mathematicians  have  learned  to  reason.    Some 
irregularities  were  detected  in  the  motion  of  the  planet 
Uranus.     There  was  a  very  strong  suspicion  that  some 
unknown  planet  caused  these  perturbations  of  Uranus, 
and   two  mathematicians,   Mr.  Adams,  an   Englishman, 
and  M.  Leverrier,  a  Frenchman,  set   themselves  sepa- 
rately to  calculation  in  order  to  find  out  the  direction 
from  which  the  disturbing  influence  must  come.     It  was 
a  remarkable  evidence  of  their  skill  that  both  gentlemen 
directed  attention  to  the  part  of  the  heavens  in  which 
the  planet  was  found,  and  put  astronomers  to  examining 
it  with  telescopes.     The  result  was  a  double  discovery, 
in  which  Leverrier  had  a  little  the  advantage  of  time, 
and  he  is  therefore  called  the  discoverer  of  the  planet, 
which  was  named  Neptune. 

162.  Let  us  go  over  the  steps  of  this  statement.     The 
merit  of  Copernicus  was  that  he  observed  nature,  just  as 
is  recommended  in  this  book,  and  reasoned  about  these 
observations  of  himself  and  others.     The  merit  of  Tycho 
Brahe  was  that  he  went  systematically  to  the  work  of 
taking  mathematical  measurements.     This  was  a  great 
step.     Then  Kepler  applied  these  measurements  to  mak- 
ing a  mathematical  statement  of  the  orbits,  the  times,  the 
distances,  the  thing  Newton  was  to  account  for.     Then 
Newton  estimated  the  force.     This  is  regarded  as  one 
of  the  greatest  achievements  of  the  human  intellect. 

163.  The  Tides. — Twice  a  day  the  waters  of  the  ocean 
move  a  short  distance  over  the  boundaries  separating 


ASTRONOMY  BY  OBSERVATION. 


sea  from  land,  and  twice  a  day  they  move  back.  This 
motion  is  called  the  tides.  When  the  water  is  rising,  it 
is  called  "  flood-tide,"  and  when  it  is  falling  it  is  called 
"  ebb-tide."  The  highest  point  reached  is  called  "  high 
water";  the  lowest,  "low  water."  High  or  low  water 
occurs  about  fifty-two  minutes  later  every  day. 

It  has  long  been  known  that  the  tides  depend  on  the 
moon's  motions ;  that  they  come  later  every  day  because 
the  moon  rises  later.  There  is  a  vast  swell,  or  tide-wave, 
on  the  half  of  the  earth  turned  toward  the  moon.  That 
we  should  account  for  this  by  the  attraction  of  the  moon 
is  natural.  But  the  waters  not  only  rise  on  the  part  of 
the  earth  under  the  moon :  they  are  at  the  same  time 
high  on  the  part  of  the  earth  turned  from  the  moon, 
while  they  are  low  only  between  these  opposite  parts  of 
the  earth.  Prof.  Guyot  thus  accounts  for  the  high  tide 
on  the  side  of  the  earth  turned  away  from  the  moon : 
"  The  waters  most  distant  from  the  attracting  body  being 
least  affected,  their  weight  is  somewhat  lessened,  and 
they  are  less  attracted  toward  the  earth  than  those  at 
the  sides.  To  restore  the  equilibrium,  the  waters  on  the 
sides,  which  exert  a  greater  pressure,  tend  to  move 
toward  the  region  of  least  attraction,  and  their  accumu- 
lation there  raises  the  surface  of  the  sea  slightly  above 
its  normal  level,  producing  the  second  or  counter  wave." 

The  attraction  of  the  sun  also  affects  the  tides,  but, 
since  he  is  much  farther  from  us  than  the  moon,  the 
effect  produced  is  much  smaller  notwithstanding  his 
greater  mass.  But  sometimes  the  attractions  of  the  sun 
and  moon  act  in  the  same  direction,  sometimes  in  differ- 
ent directions.  Twice  a  month  the  sun  and  moon  are 
on  the  same  or  opposite  sides  of  the  earth,  and  then  the 
tides  are  much  higher.  These  are  called  spring-tides. 
Twice  a  month,  when  the  moon  is  in  quadrature,  the 
attractions  of  sun  and  moon  act  at  right  angles  to  each 
other,  and  then  the  tides  are  lower  than  usual.  These 
are  called  the  neap-tides. 

The  highest  tides  occur  when  sun  and  moon,  or  both, 
are  most  nearly  vertical,  and  when  they  are  nearest  the 
earth.  Thus  they  vary  with  every  position  of  the  two 
bodies. 

The  tide  does  not  accompany  the  moon  and  sun,  but 
follows  a  little  after  them.  This  is  the  result  of  the  in- 
ertia of  matter,  which  can  not  at  once  be  set  in  motion. 
The  cause  is  the  same  that  makes  it  more  difficult  for 
horses  to  start  a  wagon  than  to  pull  it  after  it  is  once 
set  in  motion. 

The  tides  vary  in  different  places  from  terrestrial 
causes.  These  belong  to  physical  geography  rather  than 
astronomy. 

164.  Refraction. — Refraction  is  the  bending  of  a  ray 
of  light  in  passing  obliquely  from  one  medium  to  another 


of  different  density.  The  subject  belongs  to  physics, 
but  it  is  necessary  to  mention  the  effect  the  refraction  of 
the  atmosphere  has  on  the  apparent  positions  of  the 
heavenly  bodies.  The  effect  is  much  greater  nearer  the 
horizon,  since  it  depends  on  the  obliquity  of  the  rays  of 
light,  which  decreases  as  we  approach  the  zenith.  Its 
effect  is  to  make  bodies  on  the  horizon  appear  higher 
than  they  are.  In  consequence  of  refraction  we  see  the 
sun  and  stars  after  they  are  below  the  horizon.  In  the 
latitude  of  Nashville,  Tenn.,  observers  are  indebted  to 
refraction  for  a  good  view  of  the  fine  first-magnitude 
star  Canopus,  which  is  at  that  place  very  near  the  bound- 
ary of  the  circle  of  Perpetual  Disparition. 

Astronomers,  when  making  accurate  observations, 
have  great  trouble  with  refraction.  One  trouble  is  that 
the  effect  varies  with  the  density  of  the  atmosphere,  and 
thus  it  is  sometimes  difficult  to  allow  for  it. 

165.  Celestial  Measurements. — Lines  joining  the  sun 
and  earth  with  each  other  and  with  a  planet  or  the  moon  must 
form  one  line  or  a  triangle.  If  we  can  get  the  number  of  degrees 
contained  in  two  angles  of  such  a  triangle,  we  can  draw  a  tri- 
angle similar  to  it,  as  any  student  of  elementary  geometry  knows. 
Also,  the  sides  of  the  triangle  on  paper  will  have  precisely  the 
proportions  of  the  triangle  in  the  heavens.  One  of  these  sides 
is  always  the  distance  between  the  earth  and  sun  ;  another  is 
the  distance  between  the  planet  and  sun.  Thus,  by  measuring 
the  lines  on  paper,  and  getting  the  ratio,  we  compare  the  dis- 
tances of  the  earth  and  the  planets  from  the  sun.  In  this  way 
the  proportions  of  the  solar  system  were  long  ago  learned. 

The  simplest 
example  of  such 
a  triangle  is  given 
by  Venus  and 
Mercury  at  their 
greatest  elonga-  0- 
tions.  The  trian- 
gle, as  seen  in  Fig. 
39,  is  a  right  trian- 
gle. One  of  the  other  angles  is  very  easily  found  by  measure- 
ment. If  lines  be  supposed  drawn  from  the  observer's  eye  to 
Venus  and  to  the  sun,  they  form  an  angle  of  the  triangle  repre- 
sented in  Fig.  39.  At  the  time  of  her  greatest  elongation,  Venus 
can  be  seen  at  sunset  with  a  good  telescope,  and  thus  this  angle 
can  be  measured  without  difficulty.  Nothing  will  give  so  much 
reality  in  the  student's  mind  to  such  triangles  as  to  note  the 
positions  carefully  in  nature  at  the  elongation  of  Venus,  and, 
getting  the  angle  of  elongation  from  a  good  almanac  (which  al- 
ways gives  it),  to  draw  a  similar  triangle  on  paper.  If  correct- 
ly drawn,  the  lines  representing  the  distances  of  Venus  and 
the  earth  from  the  sun  have  the  proportion  of  two  to  three 
nearly. 

If  we  can  get  one  of  the  sides  of  such  a  triangle  in  miles,  it 
is  evident  we  can  get  the  others,  since  we  can  get  theig)ropor- 
tions  existing  between  the  lines.  This  is  done  by  maHng  the 


FIG.  39. 


MAP    III. 

For  Study  of  the  Stars  from   October  sad  to  January  aoth. 


345 


SOUTH 

THE    HEAVENS 

AS  SEEN 

September  22d  at  midnight,  November  yth  at  nine  o'clock, 

October  23d  at  ten  o'clock,  November  22d  at  eight  o'clock. 


CAUTION.— Be  sure  to  read  the 
directions  for  using  these  maps, 
given  in  the  Introduction,  page  5. 


MAP    IV. 

For  Study  of  the  Stars  from  April  26th  to  July  220!. 


NORTH 


SCALE    OF    MAGNITUDES 

*    *      •       •       • 

'       2         3         4         5 


SOUTH 

THE    HEAVENS 


CAUTION.— Be  sure  to  read  the 
directions  for  using  these  maps, 
given  in  the  Introduction,  page  5. 


AS  SEEN 


March  2 1st  at  midnight, 
April  2Oth  at  ten  o'clock, 


May  5th  at  nine  o'clock, 
May  2 1st  at  eight  o'clock. 


CELESTIAL  MEASUREMENTS. 


earth's  radius  the  side  of  a  triangle.  In  order  to  understand 
this,  it  is  necessary  to  discuss  parallax  a  little. 

166.  Parallax.— Let  us  suppose  that  we  note  the  direction 
of  some  body  from  us  by  drawing  a  line  to  the  center  of  its 
position  from  the  center  of  ours.  If  we  move  off  the  line,  the 
body  is  displaced  on  the  background  against  which  we  see  it. 
If  we  draw  another  line  between  the  centers  of  the  bases,  it 
forms  an  angle  with  the  former  line.  This  change  in  the  direc- 
tion of  a  body,  as  seen  from  two  different  positions,  is  called  its 
parallax.  The  retrograde  motion  of  the  planets  is  a  parallactic 
motion.  The  angle  measures  the  parallax,  or  difference  in  di- 
rection. 

From  the  previous  discussion  of  these  apparent  motions,  the 
student  knows  that  bodies  undergo  parallax  in  proportion  to 
their  nearness  to  us.  The  moon  is  the  heavenly  body  nearest 
us.  If  two  observers,  at  widely  separated  points  of  the  earth, 
observe  the  moon  at  the  same  time,  carefully  noting  her  posi- 
tion on  the  starry  sphere,  so  as  to  compare  observations,  they 
will  find  they  do  not  see  her  against  the  same  point  of  the  sphere. 
Thus,  the  moon  undergoes  parallax,  and,  by  measuring  the  an- 
gular distance  between  the  two  points  of  the  sphere,  we  measure 
the  moon's  parallax. 

When  the  moon  is  on  the  horizon,  a  line  from  her  to  the 
observer  is  a  tangent  to  the  earth.  The  earth's  radius  forms  a 
right  angle  with  it.  In  Fig.  40  the  right  triangle  moc  has  for 

FIG.  40. 


one  side  the  earth's  radius,  of,  a  line  of  known  length.  The 
observer  at  o  sees  the  moon  in  the  direction  omz.  From  the 
center  of  the  earth  the  moon  is  in  the  direction  cms.  The  dif- 
ference in  the  direction  of  these  lines  is  called  the  moon's  hori- 
zontal parallax.  It  is  the  difference  made  in  the  direction  of  a 
body  by  seeing  it  from  the  surface,  instead  of  the  center,  of 
the  earth.  The  angle  sms,  or  its  vertical  angle  cmo,  measures 
it.  This  angle  bears  such  a  relation  to  the  moon's  parallax, 
ascertained,  as  has  just  been  related,  by  observation,  that  when 
we  know  the  one  we  can  compute  the  other.  Then,  in  the 
triangle  cmo,  we  have  two  angles,  and  a  side,  c o,  in  miles. 
We  easily  find  another  side,  c  m,  which  is  the  moon's  distance 
from  the  earth's  center,  in  miles. 

167.  The  Sun's  Distance  from  the  Earth  in  Miles.— 
In  all  the  triangles  formed  by  the  sun,  the  moon,  and  a  planet, 

t  0    ••:> 


the  earth's  distance  from  the  sun  is  one  side.  If  we  know  this 
in  miles,  we  can,  from  the  proportional  triangles,  find  all  the 
other  distances  in  miles. 

Next  to  the  moon,  Mars  and  Venus  are  the  heavenly  bodies 
nearest  us.  Both  undergo  parallax  when  observed  at  widely 
separated  stations  on  earth.  When  they  are  nearest  to  us,  the 
parallax  is  greatest.  Venus  is  nearest  at  her  inferior  conjunc- 
tion, but  we  do  not  see  her  when  the  stars  are  visible,  so  we 
have  ordinarily  no  fixed  object  by  which  to  measure  the  paral- 
lax. But  twice  in  a  century  Venus  makes  a  transit  over  the 
face  of  the  sun,  and  astronomers  then  use  the  sun's  face  as  a 
background  on  which  to  estimate  the  parallax  of  Venus.  The 
points  to  be  observed  are,  the  first  and  last  contact  with  the 
sun,  and  the  distance  from  the  center.  When  this  parallax  is 
known,  the  horizontal  parallax  of  Venus  can  be  estimated,  and 
her  distance  from  the  earth  learned,  as  the  moon's  distance  was 
found. 

It  is  impossible  to  estimate  the  sun's  parallax  directly,  but 
since  the  distance  of  Venus  from  the  earth,  and  the  sun's  dis- 
tance from  the  earth,  form  sides  of  the  same  triangle,  we  can 
find  the  sun's  distance  by  knowing  that  of  Venus. 

The  last  transits  took  place  in  1874  and  1882.  The  correct 
estimate  of  the  sun's  distance  from  the  earth  is  so  important 
that  on  both  occasions  the  governments  and  scientific  men  of 
the  civilized  world  interested  themselves  in  fitting  out  expedi- 
tions to  every  quarter  of  the  globe  to  make  observations. 
There  were  years  of  preparation,  in  which  new  and  accurate 
instruments  were  made,  and  even  invented,  and  modes  of  in- 
vestigation studied. 

When  Mars  is  nearest  to  us,  he  shows  sensible  parallax.  He 
is  always  nearest  at  opposition.  Every  fifteen  years  the  in- 
crease in  the  luster  of  Mars  at  opposition  is  much  greater  than 
usual.  Fig.  32,  page  42,  shows  that  the  oppositions  of  a  superior 
planet  take  place  at  different  points  of  its  orbit.  This  variation 
in  brightness,  therefore,  leads  us  to  suspect  that  its  orbit  is  not 
a  perfect  circle.  The  orbit  of  Mars  is  very  eccentric,  and  this 
is  the  cause  of  the  remarkable  variation  in  brilliancy. 

At  long  intervals  Mars  is  at  opposition  when  he  is  at  the 
point  of  his  orbit  nearest  the  sun ;  and  the  earth  is  at  the  point 
of  hers  most  distant  from  the  sun.  Mars  then  rivals  Jupiter  in 
size.  This  occurred  in  1877.  As  the  apparent  increase  in  the 
size  of  Mars  was  due  to  his  near  approach,  it  afforded  a  good 
opportunity  for  estimating  his  parallax,  and  from  it  that  of  the 
sun.  This  was  done  with  great  care  in  1877. 

NOTE. — In  the  next  and  the  succeeding  chapters  much  use  will  be  made 
of  information  learned  by  the  telescope.  In  the  Appendix  B  there  will  be 
found  an  account  of  the  telescope,  which  is  taken  principally  from  Lockyer's 
"  Astronomy." 


ASTRONOMY  BY  OBSERVATION. 


PART    I  I. 

GENERAL  ACCOUNT  OF  THE   SOLAR  SYSTEM, 


168.  The  Solar  System  consists  of  the  sun  and  the 
heavenly  bodies  revolving  round  him  as  a  center. 

The  revolving  bodies  are  the  planets,  with  their 
moons  or  satellites,  revolving  meteors,  and  comets. 
The  earth,  on  which  we  live,  is  one  of  the  planets,  and 
therefore  the  Solar  System  is  much  more  interesting  to 
us  than  any  other  part  of  the  heavens. 

Chapter  VIII  will  treat  of  the  Sun;  Chapter  IX,  of 
the  Planets ;  Chapter  X,  of  Meteoroids  and  Comets. 


CHAPTER    VIII. 

THE    SUN. 

169.  The  sun*  subtends  an  angle  of  32',  or  a  little 
more  than  half  a  degree.  That  is,  it  would  take  about 
three  hundred  and  thirty  suns,  placed  touching  each 
other,  to  extend  across  the  sky.  The  sun's  diameter  is 
866,400  miles.  This  is  nearly  four  times  the  diameter  of 
the  moon's  orbit,  which  is  240,000  miles.  If,  therefore, 

the  center  of  the  sun 
were  placed  at  the 
center  of  the  earth, 
the  sun's  circumfer- 
ence would  be  nearly 
twice  as  far  from  us 
as  the  moon's  orbit 
now  is,  and  the  sun's 
body  would  fill  the 
whole  intervening 
space  (see  Fig.  41). 
The  sun's  volume  is 
1,305,000  times  as 
great  as  the  earth's 
volume.  But  the  sun 
is  only  about  one 

fourth  as  dense  as  the  earth.     Fig.  42  shows  the  size  of 
the  sun  as  compared  with  the  chief  planets.     The  black 

*  In  this  book  the  study  of  the  sun  has  been  deferred  until  after  the  dis- 
cussion of  the  motions  of  the  solar  system  has  been  completed,  in  order  to 
avoid  a  wide  and  long  digression  on  the  subject  of  the  sun's  telescopic  ap- 
pearance and  physical  condition.  To  ordinary  students  the  investigation  of 
the  motions  is  the  most  important  part  of  astronomy,  because  not  only  can 
all  persons  see  these  motions  in  nature,  all  must  see  them,  and  not  to  under- 
stand any  important  part  of  the  order  of  nature  brought  before  our  eyes  en- 
courages habits  of  stupidity. 


circle  represents  the  sun's  disk.     The  mass  of  the  sun  is 
about  750  times  that  of  all  the  planets  and  moons  put 

together. 

FIG.  42. 


Relative  Size  of  the  Sun  and  Planets. 

170.  Light. — The  surface  of  the  sun  is  a  hundred  and 
ninety  thousand  times  as  bright  as  would  be  a  candle- 
flame  of  the  same  size. 

It  is  a  hundred  and  FlG-  43' 

forty  times  as  bright 

as  the  calcium  or  lime 

light,  and   compares 

with  the  voltaic  arc 

as  three  and  a  half  to 

one.      On    the    sun's 

disk  the  brightness  is 

greater  at  and  near 

the    center    than    at 

the      circumference. 

This  is  owing  to  the 

greater  thickness   of 

the  sun's  atmosphere,  which  a  ray  from  the  margin  must 

penetrate  in  order  to  reach  us.     Rays  from  the  center 

cross  by  a  shorter  line,  as  is  shown  in  Fig.  43. 

171.  Heat. — Only  a  small  portion  of  the  heat  radiated 
by  the  sun  reaches  the  earth.     Prof.  Young  says :  "  If 
we  could  build  up  a  solid  column  of  ice  from  the  earth 


THE  SUN. 


53 


to  the  sun  two  miles  and  a  quarter  in  diameter,  spanning 
the  inconceivable  abyss  of  ninety-three  million  miles, 
and  if  the  sun  should  concentrate  his  power  upon  it,  it 
would  dissolve  and  melt,  not  in  an  hour,  not  in  a  minute, 
but  in  a  single  second." 

The  Spectroscope. 

172.  In  studying  the  physical  constitution  of  the  sun 
much  use  is  made  of  an  instrument  called  a  spectroscope, 
and  it  is  therefore  necessary  that  the  principles  of  its 
construction  should  be  understood  by  the  student. 

173.  When  light  from  a  luminous  gas  or  vapor  passes 
through  a  prism,  and  is  thrown  on  a  screen  in  a  darkened 
room,  it  is  found  to  produce  a  band,  or  spectrum,  con- 
sisting of  a  bright-colored  line  or  lines,  separated  by 
dark  spaces.     In  Plate  I  examples  of  such  spectra  are 
given.     Spectra  like  this  are  called  Bright-lined  Spectra. 

The  various  chemical  elements  can  be  brought  to  the 
state  of  vapor  and  made  to  produce  spectra.  Each 
substance  has  its  own  spectrum,  different  from  all  the 
others,  and  it  always  presents  the  same  appearance  when 
magnified  to  the  same  degree.  The  numbers  of  the  lines 
in  the  spectra  of  the  different  elements  differ  greatly, 
and  are  of  different  colors,  and  thus  each  element  can  be 
identified  by  its  spectrum.  If  a  compound  is  used,  its 
spectrum  will  exhibit  the  lines  of  all  its  elements. 

174.  When  the  light  from  an  incandescent  solid  or 
liquid  passes  through  a  prism,  the  spectrum  consists  of 
bands  of  color  containing  no  transverse  lines.     If,  how- 
ever, this  light  passes  through  a  luminous  vapor  before 
entering  the  prism,  it  will  consist  of  a  colored  band  with 
transverse  dark  lines  on  it ;  and  the  dark  lines  will  cor- 
respond exactly  in  position  with  the  bright  lines  which 
would  be  contained  in  the  spectrum  formed  by  passing 
the  light  of  the  luminous  gas  through  a  prism.     The 
position  of  the  lines  is  due  to  the  greater  or  smaller  re- 
frangibility  of  the  rays  producing  them,  and  thus  it  is 
evident  that   the  vapor  absorbs  rays  having  the  same 
degree  of  refrangibility  as  those  which  produce  bright 
lines.     This  is  called  a  Reversed  Spectrum. 

175.  By  comparing  the  positions  of  the  lines  in  bright 
and  dark  lined  spectra,  and  finding  the  lines  which  have 
the  same  position  in  both,  the  vapor  causing  the  dark 
lines  can  always  be  identified.     This  is  shown  in  Plate 
I,  where  the  spectrum  of  light  coming  from  the  sun  is 
compared  with  the  spectra  of  light  coming  from  various 
chemical  elements  in  the  state  of  vapor. 

The  knowledge  of  these  facts  has  given  rise  to  a  new 
method  of  chemical  analysis,  of  extreme  delicacy  and 
great  accuracy,  which  has  the  further  advantage  that  it 
can  analyze  substances  by  light  which  is  brought  from 
any  distance,  however  great.  Thus  it  can  be  applied  to 


the  light  coming  not  merely  from  the  sun,  but  also  from 
the  fixed  stars. 

It  has  given  rise  to  an  entirely  new  branch  of  astron- 
omy. Spectrum  analysis  is  thus  far  the  great  discovery 
in  the  lifetime  of  the  present  generation  of  middle-aged 
people.  The  essential  parts  of  a  spectroscope  (Figs.  44 

FIG.  44. 


A          COLLIMA  TOR 


Arrangement  of  Prismatic  Spectroscope. 

and  45)  are  —  (i)  the  prism;  (2)  the  collimator-tube, 
through  which  the  light  under  study  passes  to  the  prism ; 
and  (3)  the  telescope.  The  lens  A  collects  the  light,  and 

FIG.  45. 


the  telescope  is  used  to  magnify  the  image.     Sometimes 
the  light  is  made  to  pass  through  several  prisms. 

176.  Motion  detected  by  the  Spectroscope.  —  The 
sensations  of  light  and  sound  are  both  the  results  of  wave- 
motion.  The  undulations  of  the  air  convey  sound  :  those 
of  the  supposed  ether  convey  light.  A  sound  of  high 
pitch  is  produced  by  the  shorter  waves,  and  a  sound  of 


54 


ASTRONOMY  BY  OBSERVATION. 


low  pitch  by  the  longer  waves.  The  shorter  waves  of 
ether  produce  the  more  refrangible  colors  of  light ;  the 
longer  waves  the  less  refrangible.  Now  a  prism  separates 
and  arranges  the  colors  on  a  spectrum,  from  red  to  vio- 
let, in  the  order  of  their  refrangibility. 

It  is  a  well-known  fact  that  the  whistle  on  a  moving 
train  of  cars  has  its  pitch  gradually  raised  if  the  train  is 
approaching  the  hearer,  and  lowered  if  the  train  is  re- 
ceding from  him.  It  is  generally  true  that  sounds  mov- 
ing swiftly  from  us  gradually  fall  in  pitch  ;  those  moving 
toward  us  rise  in  pitch.  It  is  found,  when  we  examine 
the  spectrum  of  a  rapidly  moving  light,  that  the  lines 
are  displaced.  If  the  light  is  moving  from  us,  the  lines 
are  bent  toward  the  end  of  the  spectrum  at  which  the 
red  rays  appear ;  but,  when  the  light  moves  toward  us, 
the  lines  are  bent  toward  the  violet  rays  (the  least  re- 
frangible). Thus  the  spectroscope  enables  us  to  tell 
whether  a  luminous  gas  is  in  motion,  and  also  whether 
it  moves  toward  us  or  from  us.  Fig.  46  illustrates  this 
displacement  of  lines. 

177.  Solar  Spectrum. — The  spectrum  formed  by  pass- 
ing the  sun's  rays  through  a  prism  is  a  reversed  or  dark- 
lined  spectrum.  It  is  crossed  by  a  number  of  dark  lines, 
and  we  can  match  in  it  the  bright  lines  of  many  elements 
that  we  know.  Prof.  Young  has  given  a  table  of  the 
elements,  some  lines  of  which  have  been  found  in  the  solar 
spectrum. 


ELEMENTS. 

Bright  lines 
in  spectrum. 

Lines  reversed 
in  solar 
spectrum. 

Observer. 

600 
206 

89 

75 
51 
86 

71 
26 

9 
7 
15 
5 
29 

54 
27 
74 
4i 

21 
14 
64 
20 
42 

4 

460 

118 
75 
57 
33 
19 
18 
II 
9 
7 
7? 
5 
5 
4 
4 
4 
3 
3 

2 
2 
2 

12  ±  bright 
4? 

Kirchhoff. 
Thalen. 
Kirchhoff. 
Angstrom. 
Kirchhoff. 
Thalen. 
Kirchhoff. 
Kirchhoff. 
Kirchhoff. 
Kirchhoff. 
Kirchhoff. 
Angstrom. 
Lockyer. 
Lockyer. 
Lockyer. 
Lockyer. 
Lockyer. 
Lockyer. 
Angstrom. 
Lockyer. 
Lockyer. 
H.  Draper. 
Schuster. 

2,  Titanium  

4.  Manganese  

5.  Nickel  

6.  Cobalt  

7.  Chromium  

8.  Barium  

9.  Sodium  

10.  Magnesium  

12.  Hydrogen  

13.  Palladium  

15.  Molybdenum  

16,  Strontium  

17.  Lead  

18.  Uranium  

19.  Aluminium  

21.  Cadmium  

Oxygen  j8  )  

It  will  be  seen  by  this  table,  given  above,  that  the 
lines  are  not  fully  matched.     Also,  the  lines  of  a  number 


of  well-known  elements  are  not  found  at  all  in  the  sun. 
It  is  thought  that  these  results  are  due  to  the  sun's  high 

FIG.  46. 


Changes  in  tlie  C  Line  (September  22,  1870). 

temperature,  since  the  spectra  of  luminous  bodies  are 
much  affected  by  temperature. 

The  Sun's  Telescopic  Appearance  and  Physical  Constitution. 

178.  The  most  competent  authorities  now  believe  that 
the  sun  is  a  great  sphere  of  gas,  extremely  condensed  at 
the  center  by  the  weight  of  the  outer  parts. 

The  sun's  Photosphere,  or  visible  surface,  is  a  stratum 
or  coating  of  luminous  clouds  floating  in  the  sun's  atmos- 
phere, and  surrounding  the  gaseous  portion  at  the  cen- 
ter. The  Chromosphere  surrounds  the  photosphere,  and 
consists  of  very  red  flames  extending  about  six  thousand 
miles  in  every  direction  from  the  photosphere.  The 
Corona  is  a  faint  pearly  halo  resembling  the  tails  of  com- 
ets, and  it  radiates  from  the  sun  to  an  immense  distance 
in  every  direction. 

179.  The  Photosphere. — In  a  telescope  of  moderate 
power  the  photosphere  seems  to  be  composed  of  small 
incandescent     grains 

separated  by  a  some- 
what darker  medium. 
They  form  streaks 
and  groups.  With 
higher  power  of  the 
telescope  these  gran- 
ules seem  formed  of 
still  smaller  grains. 
They  must,  at  differ- 
ent times,  vary  some- 
what in  form,  having 
been  compared  by 
different  observers  to 

rice-grains  and  willow-leaves.  They  are  really  incan- 
descent clouds  floating  in  the  sun's  atmosphere,  and  com- 
posed of  metallic  vapors.  Like  the  clouds  on  the  earth's 
surface,  they  are  partially  condensed.  With  a  low 
power  of  the  telescope,  the  surface  of  the  sun  covered 


FIG.  47. 


THE  SUN. 


55 


by  the  grains  looks  like  curdled  milk.     The  light  of  the 


sun  comes  chiefly  from  these  granules. 

in  Figs.  47,  48. 

FIG.  48. 


They  are  seen 


Granules  and  Pores  of  the  Sun's  Surface.     (After  Huggins.) 


180.  Besides  the  granules,  there  are  found  on  the  sun 
brighter  streaks  looking  a  little  like  foam,  and  called 
faculas.  They  are  seen  in  Fig.  49.  The  faculas  are  por- 
tions of  the  photosphere  elevated  above  the  rest,  as  is 

FIG.  49. 


ind  Faculty.    (From  a  Photograph.) 


seen  when  they  are  on  the  edge  of  the  sun.  They  then 
project  a  little  beyond  the  circumference.  They  are 
most  common  near  the  edge  of  the  sun,  which,  as  was 
said,  is  darker  than  the  middle. 


181.  Sun-Spots. — In  addition  to  the  granules  and 
faculas  on  the  surface  of  the  sun,  there  are  spots  which 
have  been  the  subject  of  a  great  deal  of  study.  Three 
large  and  several  small  ones  are  on  the  portion  of  the 
sun's  surface  shown  in  Fig.  49.  A  sun-spot  consists  of 
two  parts,  the  umbra  and  the  penumbra  (see  Fig.  50). 
The  umbra  is  in  the  center  and  appears  dark,  but  this  is 
merely  in  contrast  to  the  brightness  of  the  granulated 

FIG.  50. 


The  Great  Sun-spot  of  1865. — /.  The  spot  entering  the  Sun's  disk,  Oct.  fth 
(foreshortened  view).  2.  Its  appearance,  Oct.  loth.  j.  Central  view,  Oct. 
I4th,  showing  the  formation  of  a  bridge,  and  the  nucleus.  4.  Its  appear- 
ance, Oct.  i6th. 

photosphere.  It  is  in  reality  filled  with  bright  clouds. 
The  penumbra  consists  of  gray  filaments  arranged  so  as 
to  radiate  from  the  center  (see  Figs.  50,  51,  52,  53).  An 
irregular  but  well-marked  outline  separates  the  penum- 
bra from  both  umbra  and  photosphere.  The  photosphere 
around  the  penumbra  is  often  intensely  bright.  Some 
penumbral  filaments  end  in  granules  of  very  bright 
matter.  These  appear  to  sink  and  dissolve,  while  others 


ASTRONOMY  BY  OSBERVATION. 


FIG.  51. 


take  their  places.  The  penumbra  seems  to  be  drawing 
in  luminous  matter  all  fhe  time.  In  a  few  of  the  spots, 
the  inner  ends  of  the  penumbral  filaments  curve  spirally, 
and  the  spots  revolve 
as  if  affected  by  a  cy- 
clone. Large  spots 
sometimes  seem  to 
have  two  different 
centers  of  cyclonic 
action.  But  the  rev- 
olution does  not  last, 
and  in  fact  such  spots 
are  not  numerous. 

Sun-spots  are  usu- 
ally circular  when  ful- 
ly formed.  Their  for- 
mation is  gradual ; 
their  coming  being 
indicated  by  faculse 
at  the  point,  and  by 

small  black  dots  which  grow  larger,  and  finally  a  spot  is 
developed   which    lasts,  on    an   average,  two   or   three 

FIG.  52- 


Spot  of  July  id,  JS66. 


A  typical  Sun-spot. 


months.  Eighteen  months  has  been  about  the  longest 
duration  of  any  sun-spot  known.  Sun-spots  are  some- 
times of  immense  size.  The  largest  spot  was  observed 
in  1858.  It  had  a  diameter  of  a  hundred  and  forty-three 
thousand  miles.  One  of  thirty  or  forty  thousand  miles 
can  easily  be  seen  with  no  further  aid  than  a  bit  of 
smoked  glass.  They  generally  come  in  small  groups. 
They  move  across  the  face  of  the  sun  which  we  see,  in 
about  twelve  or  thirteen  days,  after  which  they  disap- 
pear; but  when  another  period  of  the  same  length  has 
elapsed,  sun-spots,  which  can  unquestionably  be  iden- 
tified as  the  same,  are  found  coming  again  on  the  eastern 
part  of  the  sun.  This  is  evidently  the  result  of  the 
sun's  rotation  on  its  axis.  It  is  a  curious  fact  that  the 
motion  of  the  sun-spots  in  different  latitudes  indicates 
that  the  part  of  the  sun  near  the  equator  rotates  more 
rapidly  than  the  portion  farther  removed  from  that  cir- 
cle. At  the  equator,  the  rotation  seems  to  be  performed 
in  about  twenty-five  days. 

The  spots  are  not  found  equally  distributed  on  all 
parts  of  the  sun.  They  occur  chiefly  in  two  zones  on 
each  side  of  the  equator,  as  shown  in  Fig.  54.  These 

zones  are  between  the  latitudes 
10°  and  30°.  Sun-spots  have 
been  seen  but  once  beyond  lati- 
tude 45°. 

The  spots  seem  to  move 
across  the  sun  from  east  to  west, 
but  it  is  evident  that  the  side 
of  the  sun's  face  turned  toward 
us  corresponds  to  the  part  of  the 
earth's  surface  which  is  below 
the  horizon  of  the  person  observ- 
ing him.  It  is  evident  that  the 
part  of  the  sun's  surface  which 
we  see,  rotates  on  his  axis  in  the 
same  direction  with  the  part  of 
the  earth's  surface  which  is  below 
our  horizon.  It  is  clear  from 
this,  that  the  sun's  axial  rotation 
is  in  the  same  direction  as  the 
earth's  motion.  It  is  merely  a 
result  of  the  ambiguity  in  our 
use  of  the  words  east  and  west, 
when  we  say  the  sun  rotates 
from  west  to  east. 

It  is  evident  that  the  sun-spots 
have  a  motion  of  their  own,  as 
well  as  that  caused  by  the  sun's 
rotation. 

The  curves  made  by  the  spots, 
as  seen  in  Fig.  55,  show  that  the 


THE  SUN. 


57 


sun's  axis  is  inclined  to  the  plane  of  the  ecliptic.  That 
the  sun-spots  are  depressions  in  the  photosphere  is  shown 
by  the  changes  wrought  by  perspective  in  one  which 

FIG.  53. 


FIG.  54. 


Sun-spots  as  seen  by  Prof.  Langley. 

travels  across  the  sun's  face.  This  is  seen  in  Fig.  56. 
While  the  spot  is  on  the  western  side  of  the  sun,  nothing 
is  visible  but  the  western  side  of  the  penumbra.  As  it 

advances  east,  the 
umbra  begins  to  be 
seen  ;  then,  as  it 
passes  directly  in 
front  of  us,  we  see 
the  whole  umbra  and 
the  penumbra  all 
round  it ;  then  the 
western  side  of  the 
penumbra  goes  out 
of  sight  ;  then  the 
umbra,  and  finally 
we  see  only  the  east- 
ern side  of  the  pe- 
numbra. 

Observation  shows  that  during  periods  of  about 
eleven  or  twelve  years  there  is  an  alternate  increase  and 
decrease  in  the  activity  which  creates  sun-spots.  In 


size  and  number  they  reach  a  maximum  and  minimum. 
It  also  shows  that  the  increase  and  decrease  coincide 
with  the  increase  and  decrease  of  magnetic  disturbances 

on  earth,  •  which 
have  a  period  of 
eleven  years. 

182.  The  Chro- 
mosphere. -  This 
word  signifies  color 
sphere.  The  chro- 
mosphere consists 
of  fiery  rolling 
flames  of  a  vivid 
scarlet  color.  From 
the  chromosphere, 
red  clouds,  called 
the  "  solar  promi- 
nences," rise  into 
the  region  of  the 
corona.  Owing  to 
the  blinding  brill- 
iancy of  the  photo- 
sphere, they  can  not 
be  seen  upon  the 
sun's  disk,  and  were 
first  discovered  at 
total  solar  eclipses, 
around  the  circle  of 
sun  and  moon.  It 
was  supposed  im- 
possible to  examine 

them  at  any  other  time  but  on  these  rare  occasions,  on 
account  of  the  diffused  reflected  light  in  the  earth's 
atmosphere.  But  the  spectroscope,  which  has  revealed 
so  many  secrets,  came  to  the  aid  of  the  astronomers. 

FIG.  55. 


Apparent  Paths  of  the  Spots  across  the  Sun's  Disk,  as  seen  from  the  Earth  at 
different  times  of  the  year.  The  arrows  show  the  direction  in  which  the 
Sun  rotates. 

Dispersion  is,  as  the  name  shows,  a  separation  or 
spreading  out  of  the  rays  of  light.  A  spectroscope  of 
high  dispersive  power*  makes  the  band  of  the  spectrum 

*  The  dispersive  power  of  a  spectroscope  is  increased  by  passing  the  light 
through  a  great  number  of  prisms. 


ASTRONOMY  BY  OBSERVATION. 


Scale,  75,000  miles  to  the  inch. 


ERUPTIVE    PROMINENCES. 


Prominence  as  it  appeared  at  half-past 
twelve  o'clock,  September  7,  1871. 


As  it  appeared  half  an  hour  later,  when 
the  up-rushing  hydrogen  attained  a 
height  of  more  than  200,000  miles. 


Spikes. 


Scale,  75,000  miles  to  the  inch. 


Stemmed. 


Spot  near  the  Sun's  limb,  with  accompany- 
ing jets  of  hydrogen,  as  seen  October 
5-  1871- 


Jets. 


As  seen  at  2.45  P.  M. 


As  seen  at  3.30  P.  M. 

Three  figures,  of  the  same  prominence,  seen  July  25,  1872. 
100,000  miles  to  the  inch. 


QUIESCENT    PROMINENCES. 


Horns. 


Filamentary. 


Plumes. 


Clouds. 


THE  SUN, 


59 


FIG.  56. 


longer,  and  the  spaces  between  the  transverse  lines 
broader.  The  light  between  the  lines  is  greatly  weak- 
ened by  it,  but  the  brill- 
iancy of  the  lines  is  not 
at  all  diminished.  The 
spectrum  of  the  solar 
prominences  is  a  bright- 
lined  spectrum,  and  that 
of  the  direct  and  re- 
flected solar  light  is  a 
reversed  spectrum. 
Therefore,  by  using  a 
spectroscope  of  great 
dispersive  power,  as- 
tronomers weaken  the 
light  which  obscures  the 

Diagram  illustrating  the  Fact  that  Sun-    ,-    ,  f        ,  rvrnmi 

spots  are  Hollows  in  the  Photosphere.         "ght       ot      tne        promi- 

nences,  so  that  they  are 

now  studied  at  any  time  with  as  much  ease  as  during  a 
solar  eclipse. 

The  solar  prominences  are  of  two  kinds,  the  Quies- 
cent and  the  Eruptive  Prominences. 

183.  The  Quiescent  Prominences  change  very  grad- 
ually and  look  like  masses  of  red  clouds,  which,  when 
fully  seen,  are  found  joined  to  the  chromosphere  by 
slender  trunks,  as  seen  in  Fig.  57.  The  spectroscope 

FIG.  57. 


shows  that  they  owe  their  red  color  to  hydrogen.    These 
prominences  are  seen  all  round  the  sun. 

184.  The  Eruptive  Prominences  are  flames  which 
burst  forth  near  sun-spots,  and  are  therefore  not  found 
near  the  poles.  They  change  very  rapidly,  showing  a 
great  variety  of  transformations.  The  spectroscope 


proves  that  they  are  largely  due  to  the  vapors  of  sodium, 
magnesium,  barium,  iron,  and  titanium.  For  this  reason 
they  are  called  metallic  prominences  (see  Figs.  58,  59). 
The  chromosphere  always  shows  the  lines  of  hydrogen, 
and  sometimes  the  lines  of  the  elements  belonging  to 
the  metallic  prominences.  It  also  contains  some  lines 
which  have  not  been  identified  as  belonging  to  any  ele- 
ment that  we  know. 


FIG.  58. 


FIG.  59. 


185.  The  Corona. — (See  Figs.  60,  61,  62,  63.) — The 
corona  can  be  seen  only  when  there  is  a  total  eclipse  of 


FIG.  60. 


Corona  as  observed  by  Liais  in 


the  sun,  and,  as  this  lasts  but  for  a  few  minutes,  the  op- 
portunities for  observing  it  have  been  limited.     At  the 


6o 


ASTRONOMY  BY  OBSERVATION. 


THE  SUN. 


6l 


time  of  a  total  eclipse,  the  moon  looks  like  a  dark  sphere  in 
the  middle  of  a  halo  formed  of  "  radiant  filaments,  beams 


FIG.  61. 


Generally,  this  inner  corona  has  a  pretty  uniform  width, 
forming  a  ring  three  or  four  minutes  of  an  arc  in  width. 


Corona  of  1 87 1.     (Captain  Tttpman.) 


and  sheets  of  pearly  light.  The  portion  nearest  the  sun 
is  of  dazzling  brightness,  but  still  less  brilliant  than  the 
red  prominences  which  blaze  through  it  like  carbuncles. 


FIG.  62. 


FIG.  63. 


Corona  of  iStto.     ( Tempel.) 


Corona  of  1871.     (From  Photographs  of  Mr,  Davis.) 

separated  by  a  somewhat  definite  outline  from  the  outer 
corona,  which  reaches  to  a  much  greater  distance  and  is 
more  irregular  in  form.  Usually  there  are  several  rifts, 
as  they  have  been  called,  like  narrow  beams  of  darkness 
extending  from  the  edge  of  the  sun  to  the  outer  night, 
and  much  resembling  the  cloud-shadows  which  radiate 
from  the  sun  before  a  thunder-shower ;  but  the  edges  of 
these  rifts  are  frequently  curved,  showing  them  to  be 
something  else  than  real  shadows.  Sometimes  there  are 
long  bright  streamers,  as  long  as  the  rifts,  or  longer.  On 
the  whole,  the  corona  is  usually  4ess  extensive  and  less 
brilliant  over  the  solar  poles,  and  there  is  a  tendency  to 
accumulations  above  the  middle  latitudes  or  spot-zones." 

The  spectrum  of  the  corona  is  remarkable  for  con- 
taining one  bright  line  sometimes  called  "  1474,"  more 
generally  "  the  coronal  line,"  which  comes  from  some 
element  with  which  we  are  entirely  unacquainted ;  and 
which  when  in  vapor  (if  it  ever  is  anything  else)  is  far 
lighter  than  hydrogen,  the  lightest  substance  we  know, 
There  is  a  dark  line  corresponding  to  it  in  a  spectrum  of 
light  from  the  face  of  the  sun.  The  corona  also  shows 
faint  traces  of  hydrogen,  and  some  dark  lines  belonging 
to  the  solar  spectrum. 

The  corona  must  be  composed  chiefly  of  gas,  but  it 
probably  contains  some  minutely  divided  particles  which 
reflect  sunlight. 


62 


ASTRONOMY  BY  OBSERVATION. 


CHAPTER    IX. 

THE    PLANETS— GENERAL   ACCOUNT. 

* 

186.  The  planets  are  arranged  according  to  size  in 
two  classes,  called  respectively  the  Major  and  the  Minor 
Planets. 

187.  The  Major  Planets  are    Mercury,  Venus,  the 
Earth,    Mars,   Jupiter,    Saturn,    Uranus,   and    Neptune. 
Mercury,   Venus,   Mars,  Jupiter,  and    Saturn    were  all 
known  to  the  ancients.     Uranus  was  discovered  by  Sir 
William  Herschel  in  1781.     It  can  be  seen  by  the  naked 
eye  as  a  star  of  the  sixth  magnitude.     It  can,  however, 
hardly  be  identified  by  any  but  an  experienced  observer 
who  knows  where  to  look  for  it,  as  its  apparent  size  is  so 
small  and  it  moves  so  slowly.     The  discovery  of  Nep- 
tune has  already  been  described. 

188.  The  Minor  Planets.— Besides  the  planets  just 
named,  there  are  a  large  number  of  small  planetary  bod- 
ies revolving  round  the  sun  between  the  orbits  of  Mars 
and   Jupiter.     They   are  called   the    Minor   Planets,  or 
Asteroids. 

In  the  following  list,  the  planets  are  named  in  the 
order  of  their  distances  from  the  sun,  beginning  with 
the  one  nearest  the  center :  Mercury,  Venus,  the  Earth, 
Mars,  the  Asteroids,  Jupiter,  Saturn,  Uranus,  Neptune. 
The  distances  from  the  sun  increase  with  some  regular- 
ity, and  it  was  long  noticed  that  there  seemed  to  be  a  gap 
between  Mars  and  Jupiter,  so  it  was  thought  that  some 
unknown  planet  might  fill  it.  Early  in  the  present  cent- 
ury, four  very  small  planets  were  discovered  and  called 
Juno,  Ceres,  Pallas,  and  Vesta.  In  1843  another,  called 
Astraea,  became  known  ;  and  since  then  more  than  two 
hundred  have  been  found.  New  ones  are  often  discov- 
ered. Many  of  them  are  very  minute  bodies. 

Except  Mercury  and  Venus,  all  the  known  planets 
are  superior  planets ;  that  is,  they  revolve  round  the  sun 
in  orbits  exterior  to  the  earth's  orbit.  Their  motions 
correspond  to  the  motions  of  the  superior  planets  de- 
scribed in  Chapter  VI.  The  angle  through  which  their 
movement  appears  to  retrograde,  decreases  as  their  dis. 
tances  from  the  sun  increase. 

In  this  book,  the  forms,  volumes,  densities,  etc.,  of  the 
planets  are  described  together  under  these  respective 
heads,  because  the  student  gains  much  more  definite 
ideas  by  comparison. 

189.  Forms  of  the  Planets.  —The  earth  is  one  of  the 
planets,  and  it  will  be  best  to  speak  of  her  figure  first. 
The  earth  is  round,  or  a  sphere.     We  know  this  from  a 
variety  of  facts  :  i.  The  shadow  of  the  earth,  as  seen  on 
the  moon  when  she  is  eclipsed,  is  always  round.     2.  The 
horizon  is  always  a  circle.     If  we  are  on  a  plain,  our 


horizon  is  limited  ;  if  we  ascend  to  a  slight  elevation,  our 
view  is  more  extended  ;  and,  if  we  go  up  a  high  mount- 
ain, we  have  still  a  wider  horizon ;  but  through  all  the 
changes  our  horizon  is  still  a  circle  (see  Fig.  64).  This 
makes  it  quite  certain  that  the  earth  is  a  sphere.  3. 

FIG.  64. 


Horizons  of  the  Same  Place,  at  Different  Heights. 

When  vessels  at  sea  come  in  view  of  an  observer,  we 
see  first  the  top  of  the  mast,  then  the  upper  sails ;  next 
the  lower  sails,  and  finally  the  hull.  This  is  represented 
in  Fig.  65,  and  shows  conclusively  that  the  surface  of  the 
ocean  is  spherical. 


FIG.  65. 


Proof  of  the  Curvature  of  the  Earth's  Surface. 

But  the  earth  is  not  a  perfect  sphere.  Her  figure  is 
flattened  like  an  orange  at  the  poles.  Mathematicians 
describe  the  earth's  figure  as  an  "oblate  spheroid."  In 
order  to  understand  some  arguments  made  from  these 
facts,  the  student  is  reminded  of  twirling  a  key,  tied  to 


THE  PLANETS— GENERAL  ACCOUNT. 


FIG.  66 


a  string,  in  a  circle  round  the  hand.  That  the  key  has  a 
tendency  to  move  off  in  a  straight  line  is  shown  at  once 
when  we  let  go  the  string.  It  is  retained  in  place  by 
the  hand-grasp  and  the  cohesive  force  of  the  string.  If 
the  string  is  not  very  strong,  the  motion  may  be  rapid 
enough  to  break  it.,  Now,  there  is  a  great  deal'of  evi- 
dence to  show  that  the  earth  was  once  in  a  semi-fluid  or 
plastic  condition  from  heat;  that  is,  the  earth  was  once 
revolving  on  its  axis  with  the  force  of  cohesion  much 
weaker  than  it  now  is.  Every  particle  moved,  as  now, 
in  a  circle  like  the  key.  The  particles  about  the  surface 
of  the  equator  move  most  rapidly,  since  they  move 
through  larger  circles  in  the  same  time.  This  increases 
their  tendency  to  fly  off,  and  to  resist  all  forces  of  attrac- 
tion ;  and,  as  cohesion  was  weak  then,  they  pulled  out 
the  sphere  about  the  equator,  and  thus  the  earth  became 
an  oblate  spheroid. 

The  excess  of  matter  about  the  earth's  equator,  and 
the  attraction  of  the  sun  and  moon  for  the  parts  nearest 
them,  is  said  by  astronomers  to  cause  the  precession  of 
the  equinoxes. 

The  planets  in  different  degrees  show  this  same  pe- 
culiarity of  figure  ;  and,  as  will  be  seen,  we  have  evidence 
that  some  of  them  are  still  plastic  from  heat.  The  polar 

diameter  of  Jupi- 
ter is  five  thou- 
sand miles  shorter 
than  his  equatori- 
al diameter. 

190.  Volumes 
of  the  Planets.— 
The  word  volume 
refers  to  mere 
size,  without  re- 
gard to  weight. 
Jupiter  is  the  larg- 
est planet.  His 
diameter  is  84,000 
miles,  and  his  vol- 
ume is  thirteen 
hundred  times 
that  of  the  earth. 
Saturn  comes 
next,  with  a  di- 
ameter of  70,000 
miles,  and  nis  vol- 
ume is  seven  hun- 
dred times  great- 
er than  that  of 
the  earth.  The 
The  diameter  of 


FIG.  67. 


Earth  and  Moon. 


Comparative  Sizes  of  the' Planets. 


earth's  mean  diameter  is  7,916  miles. 

Neptune  measures  35,000  miles,  that  of  Uranus  32,000. 


Venus  is  a  very  little  smaller  than  the  earth  in  volume, 
and  her  diameter  is  only  about  300  miles  less  than  that 
of  the  earth.  The  diameter  of  Mars  is  about  4,000 
miles,  and  his  volume 
is  about  one  seventh 
that  of  the  earth.  Mer- 
cury has  a  diameter 
of  about  3,000  miles, 
and  a  volume  about 
one  sixteenth  that  of 
the  earth.  The  diam- 
eter of  the  moon  is  a 
little  more  than  2,000 
miles,  and  her  volume 
is  about  one  fiftieth 

that  of  the  earth.  These  comparative  volumes  are  illus- 
trated in  Figs.  66,  67,  68,  69. 

191.  Densities. — The  density  of  a  body  is  its  weight 
in  proportion  to  its  size,  or  in  comparison  with  an 
equal  volume  of  some  other  body.  Thus  a  pound  of 
iron  is  denser  than  a  pound  of  wood.  The  earth  is 
about  five  and  two  thirds  as  heavy  as  would  be  a  globe 

of  water  of  the  same 
size.  The  density  of 
Mercury  is  about  six 
fifths  that  of  the  earth  ; 
that  of  Venus  is  a  lit- 
tle less  than  that  of 
the  earth.  Mars  has 
only  three  fourths  the 
earth's  density.  Ju- 
piter is  lighter  than 
would  be  a  globe  of 

water  of  the  same  size,  while  Saturn  has  half  the 
density  of  Jupiter.  Uranus  is  a  little  less  dense  than 
Jupiter,  and  Neptune  than  Uranus. 

FIG.  69. 


FIG.  68. 


192.   Eccentricity  of  Orbits. — The   orbits  are  all  el- 
lipses.    We  know  that  the  earth's  orbit  is  an  ellipse  be- 


64 


ASTRONOMY  BY  OBSERVATION. 


cause  the  sun  varies  in  apparent  size ;  and  we  attrib- 
ute this  to  a  variation  of  his  distance.  The  moon 
also  varies  in  apparent  diameter.  So  do  the  superi- 
or planets  vary  in  diameter  at  different  oppositions. 
The  angle  made  by  Venus  and  the  sun  at  her  great- 
est elongations  varies  in  consequence  of  the  varying 
distance  of  Venus  from  the  sun,  and  the  same  is  true 
of  Mercury.  All  these  facts  show  that  the  orbits  are 
not  perfect  circles.  Accurate  measurement  is,  of 
course,  required  to  ascertain  the  figures  of  the  differ- 
ent ellipses. 

The  eccentricity  of  an  ellipse  is  the  degree  in  which 

FIG.  70. 


it  differs  from  a  circle.     The  orbits  of  the  earth  and 
moon  can  hardly  be  distinguished  from  circles.     That 

of  Venus  is  the  least 
eccentric  of  all. 
This  is  shown  by 
the  fact  that  the 
angle  of  her  great- 
est elongation 
differs  very  little 
from  47°.  The  al- 
manac always  re- 
ports it.  But  the 
other  inferior  plan- 
et, Mercury,  some- 
times reaches  28° 
above  the  sun,  some- 
times only  17°.  This 

makes  it  at  times  very  difficult  to>  see  Mercury.    Thus 
the  two  inferior  planets,  Venus  and   Mercury,  have  the 


least  and  the  most  eccentric  orbits  of  any  of  the  major 
planets.  Mars  comes  next  to  Mercury.  Every  fifteen 
years  he  is  much  brighter  at  opposition  than  at  the  in- 
tervening oppositions.  The  orbit  of  Jupiter  has  about 
half  the  eccentricity  of  that  of  Mars. 

The  asteroids  are  remarkable  ^or  the  very  great 
eccentricity  of  their  orbits. 

The  orbits  of  some  of  the  planets  are  illustrated  in 
Figs.  70  and  71. 

The  orbit  of  Polymnia  is  the  most  eccentric  orbit  to 
be  found  among  the  asteroids. 

193.  Inclination  of  the  Planes  of  the  Orbits  to  the 
Ecliptic. — The  ecliptic  is  the  plane  of  the  earth's  orbit, 
as  the  student  remembers.  Also  Mercury,  Venus, 
Mars,  and  Saturn  are  to  be  looked  for  very  near  the 
ecliptic.  This  is  because  the  planes  of  their  orbits  dif- 
fer so  little  from  the  ecliptic.  This  subject  becomes 
interesting  as  soon  as  we  begin  to  watch  the  planets  in 
nature.  They  are  so  near  the  great  circle  that  we  wish 
to  know  the  exact  angle  made  by  the  planes,  since  that 
marks  their  greatest  distance  from  it.  It  must  be  re- 
membered, however,  that  from  the  very  fact  that  we 
are  not  in  the  same  plane,  we  see  them  a  little  dis- 
placed from  the  positions  in  which  an  observer  at  the 
sun  would  see  them.  Uranus  crosses  the  ecliptic  at 
an  angle  of  a  little  less  than  i°;  Jupiter,  a  little  more 
than  i°  ;  Mars  and  Neptune,  less  than  2°  ;  Venus,  a  little 
more  than  3°.  Mercury  wanders  farther  from  the  eclip- 
tic than  any  major  planet.  He  is  sometimes  7°  from  it, 
and  as  the  zodiac  extends  only  8°,  he  barely  keeps 
within  it.  Mercury  deals  in  extremes.  He  is  the 
densest  planet,  the  nearest  to  the  sun,  has  the  most 
eccentric  orbit,  and  moves  farther  from  the  plane  of 
the  ecliptic  than  any  other  planet  except  the  asteroids. 
Many  of  the  minor  planets  are  sometimes  more  than 
10°  from  the  ecliptic  ;  and  one,  Pallas,  gets  as  far  as  34° 
from  it. 

Fig.  72  shows  the  angles  made  by  the  various  planes 

FIG.  72. 


The  Plane  of  the  Ecliptic  and  the  Planetary  Orbits. 

with  the  plane  of  the  earth's  orbit.  A  number  of  these 
differ  from  the  ecliptic  so  little  that  they  are  not  sepa- 
rately represented. 


s 


THE  PLANETS— GENERAL  ACCOUNT. 


•  t/t-- 


/  J     ^  **.    i?"''      " 

Distances  from  the  Sun. 


Greatest  distance — miles.     Least  distance — miles.    Mean  distance — miles. 


43,OOO,OOO 


28,OOO,OOO 


I53,OOO,OOO 

503,000,000 


I27,OOO,OOO 

457,000,000 


Mercury, 

Venus, 

The  Earth 

Mars, 

Jupiter, 

Saturn, 

Uranus, 

Neptune 

The  moon  is  240,000  miles  from  the  earth. 

Fig.  73,  which  shows  the  comparative  size  of  the  sun 
as  he  would  appear  to  an  observer  on  the  different  plan- 

FIG.  73- 


35,OOO,OOO 

67,000,000 

92,8OO,OOO 

I4O,OOO,OOO 

48O,OOO,OOO 

880,000,000 

1,770,000,000 

2,775,OOO,OOO 


The  Relative  Size  of  tke  Sun,  as  seen  from  the  Planets. 

ets,  enables  the  student  to  get  a  realizing  idea  of  the  dis- 
tances from  that  luminary. 

194.  Sidereal  and  Synodical  Revolutions. — When  we 
q 


are  interested  in  observing  the  planets,  we  very  soon 
become  familiar  with  the  meaning  of  a  sy nodical  revolu- 
tion, since  we  learn  from  it  when  to  look  for  any  planet 
in  the  evening  sky.  The  planets  are  evening  stars 
through  half  a  synodical  revolution ;  morning  stars 
through  the  other  half.  The  synodical  period  of  Mer- 
cury is  1 16  days ;  that  of  Venus,  584  days  ;  that  of  Mars, 
779  days,  or  26  months;  that  of  Jupiter,  13  months. 
The  synodical  periods  of  Saturn,  Uranus,  and  Neptune 
differ  so  little  from  a  year  that  they  seem  to  come  and 
go  with  the  fixed  stars. 

The  sidereal  periods  have  an  interest  of  another  kind. 
The  sidereal  period  of  a  planet  is  its  year,  containing  an 
entire  revolution  of  its  seasons.  These  periods  increase 
in  length  with  the  distances  of  the  planets  from  the  sun. 
They  are  as  follows : 


Mercury, 
Venus,          225 
The  Earth,  365^ 
Mars,  687 


12  years  (nearly). 


of  our  days.    Jupiter, 
Saturn, 

Uranus,      84      " 
Neptune,  165      " 

195.  Rotation  of  the  Planets.  —  The  planets  all  ro- 
tate on  axes  from  west  to  east.     We  know  this  by  the 
motion  of  spots  or  inequalities  upon  the  surface.      By 
the  curves  in  which  the  spots  move  we  learn  the  inclina- 
tion of  a  planet's  axis  to  the  plane  of  the  ecliptic,  and 
from  that  we  find  its  inclination  to  the  plane  of  its  own 
orbit.     (See  Fig.  55,  page  57.) 

Account  of  each  Planet. 

196.  Mercury.  —  We  know  little  of  Mercury,  owing  to 
his   great   nearness  to  the  sun.      He  seems  to  have  a 
somewhat  dense  atmosphere,  since,  at  the  time  of  his 
transits  over  the  sun,  his  dark  disk  has  a  dusky  bor- 
der or  ring  round  it.      Mercury   is   supposed  to  have 
no  moons.      He   receives   on  an  average    seven    times 
as  much   light  and    heat   as   the   earth.     But  he  must, 
at  opposite  points  of  his  orbit,  vary  much  in  regard  to 
both. 

197.  Venus.  —  We  have  the  same  reason  for  thinking 
that  Venus  has  an  atmosphere  that  has  just  been  given 
in  the  case  of  Mercury.     The  vapor  of  water  is  found 
in  her  atmosphere.     She  resembles  the  earth  in  size  and 
density,  but  receives  twice  as  much  light  and  heat  from 
the  sun.     We  know  little  of  her. 

198.  Mars.  —  (Fig.  74.)  In  a  very  good  telescope  Mars 
appears  to  have  his  surface  marked  by  what  appear  to 
be  islands  and  continents  of  a  dull-red  color,  and  darker 
intervening  spaces  of  greenish  hue  give  the  impression  of 
water.     By  means  of  the  spectroscope  we  find  that  there 
is  certainly  water  in  the  atmosphere  of  Mars.     There 
appears  to  be  a  great  deal  more  land  than  water  on 
Mars.    The  oceans  are  long,  with  narrow  seas  like  canals 


66 


ASTRONOMY  BY  OBSERVATION. 


Fin.  74. 


running  up  into  the  land,  so  that  water  communication 
must  be  very  general  on  Mars. 

At  the  poles  there  are  white  circles  (probably  ice- 
caps), which  di- 
minish in  size 
when  the  sun  is 
alternately  turned 
toward  them  in 
the  varying  sea- 
sons of  Mars.  We 
have  evidence 
that  the  axis  of 
Mars  is  inclined 
to  the  plane  of  his 
orbit  about  as 
much  as  that  of 
the  earth  is  ;  and 
therefore  the  sea- 
sons must  differ 
much  as  ours  do, 
except  that  they 
are  longer,  in  con- 
sequence of  the 
greater  length  of 
the  year  of  Mars, 
which  is  687  of 
our  days.  The 
day  of  Mars  is  not 
far  from  twenty- 
four  hours  in 
length.  The  red 
color  of  Mars  has 
been  attributed  to 
the  cause  which  produces  our  sunset-red,  viz.,  the  ab- 
sorption of  certain  rays  by  the  atmosphere  of  Mars.  But 
there  are  many  speculations  about  it. 

The  general  appearance  of  Mars,  the  evidence  of  land 
and  water,  his  seasons,  his  days,  make  him  appear  to  be 
a  globe  similar  to  ours,  and  not  unsuited  to  the  support 
of  life.  But  we  have  no  evidence  at  all  of  its  existence. 
Mercury,  and  especially  Venus,  come  near  us,  but  they 
are  then  so  partially  illuminated  (being  crescents)  that 
we  have  no  good  opportunity  to  see  their  surface.  Mars 
at  opposition  is  near  us,  and  in  good  position  to  be  seen, 
since  we  see  his  side  turned  to  the  sun.  He  comes  into 
this  position  only  once  in  twenty-six  months,  but  astron- 
omers are  then  very  assiduous  in  observing  him.  About 
every  fifteen  years  he  is  still  nearer.  Much  was  learned 
in  1877,  when  he  was  at  perihelion  and  the  earth  at 
aphelion,  at  the  time  of  his  opposition.  Maps  have  been 
made  of  his  surface. 

Mars  has  two  small  moons,  discovered  in  1877  by 


Mars  in  1862. 


Prof.  Hall,  and  called  Phobos  and  Deimos.  The  diame- 
ter of  the  inner  moon,  Phobos,  is  supposed  to  be  less 
than  ten  miles  in  extent.  It  is  only  about  four  thousand 
miles  from  Mars.  Our  moon  is  about  sixty  times  as  far 
from  the  earth.  Phobos  revolves  around  Mars  in  a  little 
less  than  eight  hours,  and,  as  Mars  revolves  on  his  axis 
in  about  twenty-four  hours,  it  must  move  round  Mars 
three  times  in  the  course  of  a  Martial  day.  Phobos 
must  rise  in  the  west  and  set  in  the  east.  Deimos  re- 
volves about  Mars  in  about  thirty  hours.  Its  diameter 
is  thought  to  be  less  than  forty  miles. 

199.  Jupiter. — (See  Fig.  75.)  The  axis  of  Jupiter  is 
nearly  perpendicular  to  the  plane  of  his  orbit,  and  there- 
fore there  can  not  be  much  variation  of  seasons. 

From  the  small  density  of  Jupiter,  and  the  constantly 
varying  character  of  his  surface,  it  is  believed  that  a 
large  part  of  the  planet  that  we  see  consists  of  an  atmos- 
phere filled  with  great  clouds  and  heavy  vapors.  The 
changes  are  so  great  and  sudden  that  it  is  supposed  that 
heat  must  be  the  cause  of  such  activity.  We  have  abun- 
dant evidence  that  our  own  globe  was  once  in  a  fluid 
condition  from  heat;  and  it  is  supposed  that  Jupiter  is 
still  in  that  situation,  not  having  cooled  off  sufficiently  to 
be  covered  with  a  solid  crust.  Belts  formed  of  rolling 
clouds  seem  to  stretch  across  the  disk  of  Jupiter  from 
east  to  west.  At  the  equator,  when  not  too  strongly 
magnified,  they  have  the  appearance  of  two  parallel 
belts.  Parts  of  Jupiter  show  changes  of  color,  putting 
on  a  rose  hue  between  the  belts. 

Jupiter,  like  the  sun,  is  darker  at  the  edges  than  near 
the  center,  so  that  his  moons,  when  first  seen  passing 
over  him,  appear  bright  by  contrast,  but,  as  they  ap- 
proach the  center,  the  contrast  diminishes.  There  is 
some  evidence  that  Jupiter  has  a  little  light  of  his  own, 
but  it  must  be  small,  since  the  moons  do  not  reflect  any 
of  it  when  Jupiter  is  between  them  and  the  sun.  Some- 
times there  are  spots  on  Jupiter  which  pass  away  very 
slowly.  In  1878  there  appeared  a  great  oval  red  spot 
not  far  from  the  equatorial  belts,  and  it  was  seen  at  inter- 
vals for  several  years.  There  are  indications  that,  in  the 
case  of  Jupiter  as  in  that  of  the  sun,  the  equatorial  parts 
rotate  with  a  different  velocity  from  the  parts  near  the 
poles. 

There  is  every  reason  to  think  that  Jupiter  is  in  no 
condition  to  maintain  life ;  but  he  is  of  great  interest  to 
us,  because  he  seems  to  be  in  the  condition  in  which  our 
earth  was  while  undergoing  the  changes  which  fitted  it 
to  be  the  abode  of  men  and  animals. 

Jupiter  has  four  large  moons.  The  largest  has  a  di- 
ameter of  about  3,700  miles,  being  nearly  as  large  as  the 
planet  Mars.  The  smallest  is  about  the  size  of  the  earth's 
moon.  The  shortest  revolution  around  Jupiter  takes  a 


THE  PLANETS— GENERAL  ACCOUNT. 


67 


little  less  than  two  days,  and  the  longest  nearly  seventeen 
days.  When  Jupiter  passes  between  his  satellites  and 
the  sun,  they  are  eclipsed  and,  of  course,  darkened.  But 


FIG.  75. 


Jupiter  can  pass  between  us  and  his  satellites  without 
passing  between  them  and  the  sun.  They  are  then  said 
to  be  occulted,  because  they  are  hidden  from  us ;  but 
they  are  not  darkened. 

When  a  moon  passes  between  Jupiter  and  the  sun 
it  casts  a  shadow  on  him.  The  bright  side  of  the  moon 
is  turned  toward  us,  and  is  seen  to  pass  over  the  planet's 
face.  This  is  called  a  transit  of  the  moon.  As  we  are 
not  often  in  a  straight  line  with  the  planet  and  a  moon, 
we  usually  see  both  the  bright  spot  and  the  shadow. 
(See  Fig.  75.) 

The  axis  of  Jupiter  is  very  little  inclined  to  the  plane 
of  his  orbit,  and  the  orbits  of  his  moons  are  very  nearly 
in  the  plane  of  his  equator,  so  that  they  are  all,  except 
one,  eclipsed  at  every  revolution.  If  there  were  an  ob- 
server on  Jupiter,  he  would  see  in  a  year  over  four  thou- 
sand eclipses  of  the  sun,  and  about  the  same  number  of 
eclipses  of  the  moon. 

The  moons  of  Jupiter  can  be  seen  through  a  very 
small  telescope. 

200.  Saturn. — The  axis  of  Saturn  is  much  inclined  to 
the  plane  of  his  orbit. 


The  telescopic  appearance  of  Saturn  has  long  been 
of  great  interest  on  account  of  his  rings.  Figs.  76,  77, 
give  pictures  of  Saturn  and  his  rings.  The  rings  are 
thin  and  flat,  and  lie  on  the  plane  of  Saturn's 
equator.  They  consist  of  three  parts:  i.  A 
dusky  ring  nearest  the  planet,  and  therefore 
in  a  position  where  it  can  not  reflect  much 
sunlight  upon  us.  It  is  therefore  difficult  to 
see  it.  The  outline  of  the  planet  has  been 
seen  through  this  interior  dark  ring.  2. 
Next  to  this  dusky  ring  there  is  a  bright  ring. 
3.  Outside  of  this  bright  ring,  there  is  an- 
other, not  quite  so  bright,  but  still  brighter 
than  the  dusky  ring ;  and  between  the  two 
outer  rings  there  is  an  open  space  which  is 
black  in  the  picture.  The  inner  edge  of  the 
dusky  ring  is  about  twenty  thousand  miles 
from  the  planet's  surface.  The  rings  are  in 
motion  round  the  globe ;  and,  with  the  axis, 
are  inclined  about  27°  to  the  plane  of  the 
planet's  orbit.  In  moving,  both  axis  and 
rings  keep  this  inclination. 

The  rings  of  Saturn  are  now  generally 
believed  to  consist  of  a  multitude  of  small 
satellites  so  close  together  that,  like  a  swarm 
of  bees,  they  seem  at  a  distance  to  be  con- 
tinuous. In  the  dusky  ring,  the  particles  are 
supposed  to  be  farthest  apart.  The  interior 
dark  ring  is  sometimes  called  "  the  crape 
ring."  The  diameter  of  the  outermost  ring 
is  more  than  one  hundred  and  fifty  thousand  miles. 

From    the  movements  of  Saturn  and  the  earth,  his 

FIG.  76. 


Saturn  and  the  Earth — Comparative  Size, 


68 


ASTRONOMY  J3Y  OBSERVATION. 


FIG.  77. 


ring  is  turned  in  a  great  variety  of  positions  as  regards 
the  observer.     These  are  called  the  phases  of  the  rings, 

and  they  are 
shown  in  Fig.  78. 
The  rings  and 
their  phases  may 
be  seen  in  a  tele- 
scope of  small 
power,  but  a 
good  view  of  the 
three  rings  can 
be  had  only  in  a 
telescope  of  high 
power.  As  Sat- 
urn is  29^  years  in  making  a  complete  revolution 
round  the  sun,  he  takes  the  whole  period  to  show  all  the 
phases.  At  two  opposite  points  in  Saturn's  orbit,  the 
ring  is  turned  edgewise  toward  us,  and  then  it  disap- 

FIG.  78. 


Saturn  with  the  Xorth  Surface  of  its  Rings  pre- 
sented to  the  Earth. 


FIG.  79. 


Different  Appearances  of  Saturn  's  Rings. 

pears,  except  in  the  most  powerful  telescopes  ;  and  in 
them  it  looks  like  a  thin  line  of  light.  The  satellites  of 
Saturn  are  nearly  in  the  plane  of  the  ring,  and  when  it 

has  this  appearance  they  are 
seen  moving  on  its-  edge  "  like 
golden  beads  on  a  silver 
thread."  The  disappearance 
of  the  ring  shows  its  very 
great  thinness.  This  phase 
is  shown  in  Fig.  79. 

Saturn's  sphere  seems  to 

fa    very    much    in   the    COndi- 

.  J        .  . 

tion  of  Jupiter.  It  is  covered 
with  clouds  in  which  some 
faint  traces  of  belts  are  distinguished,  and  is  probably 
still  hot  and  the  seat  of  great  activity.  But  Saturn 


Appearance  of  Saturn  when  the 
Plane  of    its    Rings    passes 

through  the  Earth. 


is  so  far  away  that  it  is  difficult  to  know  much  about 
him. 

Saturn  has  eight  moons.  The  largest  of  these,  Titan, 
is  larger  than  the  planet  Mercury,  and  can  be  seen  in 
very  small  telescopes.  But  they  are  not  all  visible  ex- 
cept in  telescopes  of  the  largest  size. 

201.  Uranus  and  Neptune. — Nothing  is  known  in  re- 
gard to  their  physical  features.     In  a  large   telescope, 
Uranus  is  said  to  have  a  sea-green  appearance.     Uranus 
receives  ^^  as  much  light  and  heat  as  the  earth  ;  while 
Neptune  receives  only  ld\6  as  much. 

Uranus  has  four  moons,  whose  orbits  are  inclined 
nearly  80°  to  the  plane  of  the  planet's  orbit.  Neptune 
has  one.  The  satellites  of  Uranus  and  Neptune  have 
one  remarkable  peculiarity  :  they  revolve  around  their 
planet  from  east  to  west;  all  other  planets  and  satel- 
lites revolve  from  west  to  east. 

202.  The  Moon. — That  the  moon  has  no  appreciable 
atmosphere   is  clearly  shown  in  several  ways.     At  half- 
moon,  the  diameter  which   forms  one  boundary  is  the 
dividing  line  between  day  and  night  on  the  moon.     If 
there  were  an  atmosphere,  there  would  be  some  indica- 
tion of  twilight  near  this  line ;  but  there  is  none.     The 
shadows  of  the  mountains  are  pitchy  black,  showing  no 
trace  of  the  diffused  light  which  would  result  from  an 
atmosphere.     Besides,  if  the  moon  had  any  atmosphere, 
we  should  expect,  at  a  solar  eclipse,  that  her  edge  would 
be  surrounded   by  a  dusky  ring  or  border,  such  as  are 
seen  around  Venus  and  Mercury  when  making  a  transit 
across  the  sun's  face.     But  there  is  nothing  of  the  kind. 
Also,  in  the  moon's  course  through  her  orbit,  she  often 
passes  over  or  occults  a  star.     If  she  had  any  atmosphere, 
we  should  proba- 
bly see  these  stars  FIG.  80. 

become  dimmed 
just  before  disap- 
pearing :  but  they 
seem  suddenly 
blotted  out.  As- 
tronomers have 
made  careful  ob- 
servations to  see 
whether  there 
were  any  traces 
of  refraction.  The 
conclusion  is,  that 
the  moon  has  no 
appreciable  atmo- 
sphere. 

There  never  is 
any  appearance  of 
clouds  passing  over  the  moon,  except  clouds  near  the 


. 
.   , 

I 


Moon  at  the  First  Quarter.     (From  Photographs 
taken  by  Prof.  H.  Draper,  New  York.) 


THE  PLANETS— GENERAL  ACCOUNT. 


69 


FIG.  81. 


Moon  Scenery. 

is,  thus  far,  an  absence  of  indication  of 


earth.     There 
water. 

The  moon's  surface,  to  an  observer 
with  the  naked  eye,  seems  spotted 
with  dusky  patches,  in  which  imagi- 
nation sometimes  sees  a  resemblance 
to  a  human  face  popularly  called  "  the 
man  in  the  moon."  Under  a  telescope 
of  low  power,  the  dusky  patches  ap- 
pear smooth,  but  with  higher  power, 
elevations  and  depressions  become 
visible.  The  moon's  face  seems  to 
be  thickly  pitted  with  the  craters  of 
extinct  volcanoes.  Many  of  them  have 
central  cones  which  have  every  ap- 
pearance of  having  arisen  from  erup- 
tive action,  which  would  have  great 
power  on  the  moon,  since  the  force  of 
gravity  would  be  less  than  on  earth, 
owing  to  the  moon's  smaller  mass. 
Some  of  the  craters  are,  however, 
different  from  those  of  any  volcanoes 
that  we  know,  appearing  to  be  mere 
plains  surrounded  by  irregular  circu- 
lar walls.  In  the  formations  on  the 
moon  there  is  a  very  general  tendency 
to  circular  shape.  The  greater  num- 
ber of  the  craters  are  depressed  below 
the  surface,  but  some  are  hollowed 
out  in  elevations.  In  some  places 


they  stand  singly  on  the  plain ;  in  others  they  are 
crowded  and  heaped  upon  one  another ;  sometimes  there 
are  small  craters  on  a  plain  which  is  surrounded  by  the 
wall  of  a  large  crater.  They  are  of  all  sizes,  from  the 
small  one  just  visible  to  us,  to  the  great  one  with  a  diam- 
eter of  a  hundred  and  fifty  miles.  The  great  crater 
Ptolemy  incloses  a  space  equal  to  the  State  of  Massa- 
chusetts. These  formations  are  generally  regarded  as 
due  to  volcanic  origin,  but  it  is  not  believed  that  there 
are  now  any  active  volcanoes  on  the  moon.  The  moon 
has  been  compared  to  a  burned-out  cinder. 

The  inequalities  of  the  moon's  surface  are  best  shown 
at  or  near  her  quadrature.  The  line  which  separates 
light  from  darkness  is  called  the  terminator.  On  this 
line  the  sun  is  all  the  time  rising  or  setting,  and  there- 
fore long  shadows  are  thrown  which  bring  out  details 
and  show  the  heights  of  mountains.  (Something  of  this 
irregular  appearance  of  the  terminator  can  be  seen  at 
quadrature  without  a  telescope.)  Besides  this,  there 
are  elevated  portions  which  catch  the  light  both  before 
sunrise  and  after  sunset.  These  effects  can  be  seen 
very  plainly  with  a  telescope  of  even  moderate  power. 
Fig.  80,  showing  the  shadows,  and  also  the  light  on  the 

FIG.  82. 


Moon  Scenery. 


ASTRONOMY  BY  OBSERVATION. 


elevated   points,  is  a  reduced   copy  of  one  of  Ruther- 
ford's photographs  of  the  moon. 

There  are   also  chains  of   mountains  on  the  moon. 

FIG.  83. 


Moon  Scenery. 

Their  heights  have  been  ascertained  by  means  of  their 
shadows ;  and  it  is  found  that  in  proportion  to  the  size 
of  the  moon,  her  mountains  are  higher  than  those  on  the 
earth.  There  are  chains  which  have  been  named  re- 
spectively the  Alps,  the  Apennines,  and  the  Caucasus. 
Fig.  8 1  show^the  region  of  the  lunar  Alps,  with  the 
great  crater  •  Plato.  The  diameter  of  Plato's  ring  is 
about  seventy  tniles,  and  the  mountains  surrounding  it 
are  five  or  six  thousand  feet  high.  To  the  left  of  Plato 
there  is  a  remarkable  valley,  the  valley  of  the  Alps.  It 
is  as  level  as  if  it  were  a  roadway  made  by  engineers; 
but  it  is  bounded  by  very  tall  mountains.  It  is  six  miles 
wide  and  seventy-five  miles  long.  The  sun  is  on  the 
left  of  the  picture,  and  the  mountains  throw  long  shad- 
ows on  the  right. 

Fig.  82  is  a  representation  of  the  lunar  Apennines  and 
the  great  crater  Archimedes.  These  mountains  rise  to 
nearly  eighteen  thousand  feet.  On  the  plain  there  are 
black  lines  representing  the  chasms,  cracks,  or  canals, 
which  form  a  curious  feature  of  the  moon's  surface. 
Some  of  them  are  a  hundred  miles  long.  They  are  sup- 
posed to  be  of  volcanic  origin,  like  so  many  other  feat- 
ures of  the  moon's  surface. 


Fig.  83  shows  an  ideal  lunar  landscape,  taken  from 
the  work  of  Nasmyth  and  Carpenter  on  the  moon. 

There  is  another  very  curious  feature  of  the  moon's 
surface  seen  when  she  is  full,  and 
when,  of  course,  the  perpendicu- 
lar rays  of  the  sun  shine  on  her. 
There  are  seen,  radiating  from 
some  of  the  craters,  bright 
streaks,  which  run  over  the  sur- 
face of  the  moon  for  hundreds  of 
miles,  crossing  mountains  and 
valleys  without  seeming  to  be 
stopped  by  any  obstacles  what- 
ever. Fig.  84  shows  the  full 
moon  and  these  bright  streaks. 
They  radiate  especially  from 
three  great  craters,  Tycho,  Co- 
pernicus, and  Kepler.  They  do 
not  appear  to  be  elevations  or 
depressions  on  the  moon's  sur- 
face. It  has  been  well  said, 
"  They  look  as  if,  after  the  whole 
surface  of  the  moon  had  received 
its  final  configuration,  a  vast 
brush  charged  with  a  whitish 
pigment  had  been  drawn  over 
'  the  globe,  leaving  its  trail  upon 
everything  it  touched,  but  ob- 
scuring nothing."  An  effort  has 

been  made  to  account  for  these  appearances  by  sup- 
posing that  the  moon,  in  cooling,  suddenly  cracked ; 
that  these  cracks 


afterward  became 
filled  with  melted 
lava,  which,  when 
cool,  presented  a 
smooth  surface  ca- 
pable of  reflecting 
light.  This,  of 
course,  is  not  much 
more  than  conjec- 
ture. 

It  is  supposed 
that  the  moon  rep- 
resents a  body  like 
the  earth,  in  a  much 
more  advanced 
stage  of  cooling 
than  the  planet  on 
which  we  live.  It 
stage  when  it  can 
that  we  know. 


FIG.  84. 


Full  Moon.     (From  Photographs  taken  by  Prof. 
H.  Draper,  New  York.) 

has,  astronomers  think,  reached  the 
no  longer  support  any  form  of  life 


METEOROIDS  AND   COMETS. 


CHAPTER    X. 

METEOROIDS    AND    COMETS. 

203.  Shooting-Stars. — When  we  are  out  of  doors  after 
dark  on  a  clear  evening,  we  frequently  see  what  appear 
to  be  stars  moving  swiftly  across  the  sky  and  vanishing ; 
sometimes  leaving,  for  a  few  seconds,  a  long  train  of 
light,  and  sometimes  breaking  into  pieces  without  any 
noise.     These  bodies  are  called  shooting-stars. 

204.  Meteors. — Occasionally   we  see  larger  moving 
bodies  giving  a  brilliant  light,  which  in  some  instances 
is  bright  enough  to  illuminate  the  whole  heavens.     Some 
of  these  explode  with  a  loud  noise.     These  larger  shoot- 
ing-stars are  commonly  called  meteors,  though  the  name 
applies  in  strictness  to  both  classes. 

205.  Aerolites. — Besides  this,  stony  or  metallic  bodies 
are,  at  rare  intervals,  known  to  fall  through  the  air, 
penetrating  a  short  distance  into  the  earth,  and  being 
hot  if  found  soon  after  the  fall.      A  very  few  of  these 
stones  are  large  bodies.     One  in  the  cabinet  of  Yale  Col- 
lege weighs  nearly  a  ton.     Sometimes  they  fall  in  num- 
bers.    These  falling  bodies  are  called  aerolites.     They 
are  composed  of  chemical  elements  well  known  on  earth, 
but  sometimes  in  combinations  not  seen    here    except 
under  circumstances  which  furnish  good  evidence  that 
they  have  fallen  from  the  skies. 

206.  Composition.  —  It   is   now   believed   that   these 
bodies  can  be  distinguished  by  their  composition  when 
there  is  nc  other  evidence  of  their  fall.     They  always 
contain  iron  in  the  metallic  state,  which  is  very  rarely 
found  on   earth.     They  are  called   meteoric  irons  and 
meteoric  stones,  according  as  they  are  composed  largely 
of  metallic  iron,  or  as  they  contain  a  larger  proportion 
of  other  elements.     They  als®  have  in  them  compounds 
of  iron  not  known  on  earth.     Meteoric  iron  has  a  crys- 
talline  structure,  and   aerolites   in   general    look   as   if 
they  had  been  melted  to  some  distance  below  their  sur- 
faces. 

207.  Origin. — It  is  now  thought  that  a  large  number 
of  small  particles  and  masses  of  matter  revolve  round 
the  sun,  and,  coming  within  the  sphere  of  the  earth's 
attraction,  they  are  brought  into  forcible  contact  with 
our  atmosphere.     The  collision  produces  heat  and  like- 
wise light,  and  in  some  cases  the  effect  is  great  enough 
to  produce  explosion.     The  atmosphere  is  very  attenu- 
ated matter,  but  the  velocity  is  so  great  that  the  result 
is  like  that  of  striking  flint  with  far  less  velocity. 

To  these  bodies,  called  meteoroids,  are  due  shooting- 
stars,  meteors,  and  aerolites,  which  thus  all  belong  essen- 
tially to  the  same  class.  Meteoroids  are  supposed  to  be 
exceedingly  numerous.  It  has  even  been  estimated  that 
eight  million  pass  through  our  atmosphere  in  twenty-four 


FIG.  85. 


hours.     The  visible  path  of  shooting-stars  is  usually  be- 
t''cen  fifty  and  seventy-five  miles  from  the  earth. 

208.  Meteoric  Showers. — There  are  also  periodical 
showers  of  shooting-stars.  About  November  I4th  there 
is  a  noticeable  shower  every  year.  The  meteors  seem 
to  radiate  from  the  constellation  Leo,  and  therefore  they 
are  called  the  Leonids.  The  radiation  is  apparent.  The 
small  bodies  move  in  parallel  lines,  and  they  seem  to 
diverge  from  Leo  because  all  parallels  seen  a  long  way 
off  appear  convergent.  The  orbit  of  these  meteors  is 
exhibited  in  Fig.  85.  They  evidently  move  in  their  orbit 
retrograde,  or  from  east  to  west.  There  must  be  a  dis- 
tribution of  meteors  along 
the  whole  line,  since  they 
are  seen  every  year,  but 
every  thirty  -  three  years 
there  is  an  unusual  exhibi- 
tion of  them,  and  therefore 
it  is  supposed  a  great  num- 
ber are  collected  in  one 
part  of  their  orbit.  The 
earth  crosses  this  orbit 
every  year,  but  only  en- 
counters the  great  body  of 
meteors  every  thirty-three 
years,  because  that  is  their 
period  of  revolution.  In 
1833  there  was  a  striking 
display  of  these  meteors  in 
the  United  States.  The 
sky  was  covered  with  lines 
of  light  in  every  direction, 
and  great  alarm  was  ex- 
cited among  ignorant  people.  The  last  great  exhibition 
occurred  in  1866,  but  it  did  not  equal  thfi  of  1833.  It 
is  supposed  that  the  dense  mass  is  of  suqjftxtent  that  the 
earth  gets  into  some  part  of  it  for  three  successive  years. 
There  was  a  lesser  recurrence  of  the  shower  of  1866  in 
the  two  following  years.  The  November  meteors  do 
not  generally  explode.  They  are  small  bodies. 

Another  annual  shower  of  less  brilliancy  comes  in 
August,  and,  as  these  seem  to  radiate  from  the  constella- 
tion Perseus,  they  are  called  the  Perseids.  It  is  sup- 
posed that  the  earth  passes  through  the  orbit  of  the 
Perseids  in  August.  Since  there  is  no  variation  in  dif- 
ferent years,  it  is  supposed  that  the  Perseids  are  pretty 
regularly  distributed  along  their  orbit.  The  perihelion 
points  of  the  orbits  of  both  Leonids  and  Perseids  touch 
the  earth's  orbit.  The  orbit  of  the  August  meteors 
reaches  far  beyond  Neptune,  and  it  is  thought  they 
take  one  hundred  and  twenty  years  to  make  a  revolu- 
tion. 


ASTRONOMY  BY  OBSERVATION. 


FIG.  86. 


There  are  other  meteor  groups,  but  these  are  the 
most  important  and  well  known. 

Comets. 

209.  Description. — When  comets  are  first  seen  with 
the  telescope,  before  they  come  near  enough  to  be  visi- 
ble to  the  naked  eye, 
they  look  (as  do  the 
.nebulas  hereafter  to 
be  described  among 
the  fixed  stars)  like 
small  clouds.  They 
can  be  distinguished 
from  the  nebulas  only 
by  their  motion. 
There  are  many  com- 
ets called  telescopic 
comets,  because  they 
can  not  be  seen  by 
the  naked  eye.  Their 
approach  to  the  sun 
makes  them  large  and 
conspicuous.  In  or- 
der that  we  may  see 
them,  especially  with 
the  naked  eye,  their 
nearest  point  to  the 
sun  must  be  not  very 
far  from  the  earth's 
orbit. 

210.  Parts  of  Com- 
ets.— Comets  which  can  be  seen  without  a  telescope 
have,  when  near  the  sun,  three  parts.  They  have  a  nu- 
cleus, which  is  very  like  a  bright  star,  and  which  is  sur- 
rounded by  a  cloudy,  shining 
envelope  called  their  Coma. 
They  have  also  a  tail,  which  is 
a  long  stream  of  light  extending 
from  the  coma  (see  Fig.  86). 
Their  approach  to  the  sun  de- 
velops these  parts.  As  they 
draw  near  that  luminary,  they 
throw  out  jets  toward  the  sun 
which  seem  to  be  alternately 
attracted  and  repelled  by  him. 
The  appearances  are  very  like 
those  exhibited  by  bodies  in  op- 
posite states  of  electricity.  It 
has  also  been  noticed  that  large 
comets  in  the  neighborhood  of 
the  sun  throw  off  from  the  jets, 

.    •"        '       Donati  s    Comet  (showing  the 
toward    the  Sun,  a  Succession  Of  Head  and  Envelopes). 


Comet  of  1264. 


FIG.  87. 


FIG. 


apparently  vaporous  envelopes.  These  were  observed 
with  great  care  in  the  case  of  Donati's  comet,  which 
appeared  in  1858.  Fig.  87  shows  these  envelopes.  Do- 
nati's comet  was  one  of  the  most  brilliant  of  modern 
times  (see  Fig.  88).  It  came  so  near  the  earth's  orbit 
that,  had  the  earth  been  on  the 
same  side  of  the  sun,  it  must 
have  passed  through  the  com- 
et's tail.  The  same  envelopes 
were  seen  in  Coggia's  comet 
in  1874. 

211.  Tails  of  Comets. — As 
comets  draw  near  the  sun,  their 
tails  are  developed  with  great 
rapidity  on  the  side  turned 
away  from  the  sun.  Thus, 
when  approaching  the  sun,  the 
tail  follows  the  nucleus  and 
coma,  but  in  receding  from  the 
sun  the  nucleus  and  coma  fol- 
low the  tail.  The  tails  of  com- 
ets are  of  very  various  lengths, 
some  extending  more  than  half 
across  the  heavens  above  the  horizon.  Their  tenuity  is 
very  great,  very  faint  stars  being  seen  through  them. 
The  earth  is  believed  to  have  passed  through  the  tail  of 

FIG.  8g. 


Donati's  Comet  (general  view). 


Comet  of  i8bi. 

a  comet  in  1861,  without  the  fact  being  known  to  its  in- 
habitants. This  was  a  very  brilliant  comet,  fan-shaped. 
It  is  shown  in  Fig.  89.  The  comet  of  1744  had  five  tails 
(see  Fig.  90). 

212.  Origin  of  Comets. — Comets  are  supposed  to  come 
from  the  stellar  spaces  beyond  the  solar  system,  and  to 
be  drawn  into  the  sphere  of  the  sun's  attraction.  Some 


METEOROIDS  AND   COMETS. 


73 


move  in  ellipses  and  are  called  periodical  comets,  since 
they   return  ;    while  others   appear  to  move  in  curves 


FIG.  go. 


Comet  of  1744. 

which  do  not  reach  the  point  from  which  they  started.* 
Whether  the  figure  is  an  ellipse  depends  on  its  velocity 
in  proportion  to  its  distance  from  the  sun.  Where  the 
ratio  of  the  velocity  to  the  distance  is  small,  the  figure 
is  an  ellipse,  and  the  degree  of  eccentricity  of  the  ellipse 
depends  upon  the  same  proportion.  But  comets  are  apt 
to  have  their  speed  increased  or  diminished  by  the  at- 
traction of  the  planets  which  they  pass,  and  thus  their 
orbits  become  changed.  The  attraction  of  the  greater 
planets  produces  decided  effect  in  changing  the  orbits 
of  the  comets  to  ellipses.  In  that  case  the  aphelion 
points  of  their  orbits  are  found  not  very  far  from  the 
orbit  of  the  planet.  There  are  a  number  of  comets  thus 
connected  with  each  one  of  the  larger  planets. 

213.  Return  of  Comets. — The  first  comet  of  which  the 
return  was  successfully  foretold  is  called  Halley's  comet. 
He  saw  it  in  1682,  and  found  that  its  orbit  was  about  the 
same  as  that  of  two  comets  which  had  previously  ap- 
peared, one  in  1531,  the  other  in  1607.  The  orbit  of  the 
latter  had  been  investigated  by  Kepler.  The  comet 
made  its  appearance  a  little  later,  but  it  was  shown  by 
another  astronomer  that  its  delay  was  occasioned  by  the 
attraction  of  Jupiter  and  Saturn.  It  was  shown  to  be 
identical  with  a  comet  seen  and  recorded  in  1066,  1456, 
and  1531.  It  returned,  according  to  Halley's  prediction, 
in  1759,  and  was  last  seen  in  1835,  when  it  was  very 
brilliant  and  excited  great  interest.  In  1456,  it  caused 
much  alarm,  as  this  was  soon  after  the  Turks  took  Con- 
stantinople and  threatened  the  rest  of  Europe.  It  was 
this  comet  that  caused  the  prayer,  "  From  the  comet, 

*  These  curves  are  known  to  the  students  of  conic  sections  as  parabolas 
and  hyperbolas. 
10 


the  Turk,  and  the  devil,  good  Lord,  deliver  us."     It  was 
60°  in  length,  and  was  thought  to  resemble  a  saber. 

214.  Meteors  and  Comets. — There  are  a  number  of 
facts  showing  a  close  connection  between  comets  and 
meteors.     There  is  a  comet  which  has  the  same  orbit  as 
the  November  meteors,  while  another  has  that  of  the 
August  meteors.      Biela's  comet  furnishes  further  evi- 
dence of  this  relation.     This  is  a  periodic  comet,  and 
when  it  came  again  -in   1845   it  was  found,  after  its  ap- 
pearance, to  have  divided  into  two  parts,  of  which  one 
disappeared  before  the  other.     In  1852   both  returned, 
and  were  found  still  farther  apart.     After  their  disap- 
pearance, they  were  not  again  seen.     But.  in   1872,  the 
comet  was  due,  and  on  November  27th,  the  time  when 
the  earth's  orbit  crosses  that  of  the  comet,  there  was  a 
great  shower  of  meteors,  which  seemed  to  come  from 
the  part  of  the  sky  where  the  comet  would  have  been 
situated.     (See  Note,  p.  74.) 

It  is  thought  that  the  nucleus  of  a  comet  may  consist 
of  a  collection  of  meteoroids.  Through  some  telescopic 
comets,  stars  can  be  seen,  and  in  this  case  the  meteoroids 
must  be  small  and  far  apart.  But  in  large  comets  they 
must  be  very  dense,  if  the  nucleus  is  not  solid.  The  tail 
and  coma  are  produced  by  vaporization  as  they  draw 
near  the  sun.  In  1843  there  was  a  very  brilliant  comet 
clearly  visible.  Its  tail  stretched  through  65°.  It  passed 
very  near  the  sun,  being  within  his  exterior  atmosphere, 
and,  while  thus  near,  passed  half  round  the  sun  in  two 
hours,  revolving  its  tail  through  180°  in  that  short  time. 
From  this  it  would  appear  that  the  matter  of  which  the 
tail  is  made  is  all  the  time  changing,  like  that  of  smoke 
from  a  chimney  or  vapor  from  a  kettle. 

215.  Comets  and   the  Spectroscope. — Comets  have 
been  examined  in  the  spectroscope ;  the  brilliant  comet 
of  June,  1 88 1,  being  subjected  to  thorough  study.    They 
show   a   spectrum  in  which  are   indications  of   hydro- 
carbon vapors.     Their  light  is  probably  due  partly  to 
reflection   of   the   sun's   light,  and    partly  to   these  va- 
pors when  acted  upon  by  an  electric  discharge  passing 
through  them. 

216.  Numbers  of  Comets.— There  have  been  about 
five  hundred  comets  seen  by  the  naked  eye   since  the 
beginning  of   the  Christian   era.     About  two  hundred 
telescopic  comets  have  been  seen  since  the  instrument 
was  invented.      Doubtless  a  much   larger  number  has 
existed  beyond  the  limits  of  our  observation,  which  are 
very  narrow.     There  are  accounts  of  comets  in  ancient 
times,  but  the  description  is  so  colored  by  the  alarm 
they  excited  that  it  can  not  be  trusted.     There  was  a 
great  comet  visible   before  the   assassination  of  Julius 
Caesar,  43  B.  c.     It  was   seen  for  several    hours  before 
sunset.     It  was  supposed    by  Halley  to  be  the  same 


74 


ASTRONOMY  BY  OBSERVATION. 


FIG.  91. 


/ 


.  Comet  °f  i8i>- 

comet  which  appeared  in  1680,  when  its  orbit  was  in- 
,  vestigated  by  Sir   Isaac  Newton.     A  very  remarkable 

comet  was  seen  in  1811,  just  before  the 
>  -^.invasion  of  Russia  by  Napoleon  Bona- 
parte.  It  was  very  brilliant,  and  was 
seen  for  seventeen  months.  (See  Fig. 
91.) 

217.  The  Zodiacal  Light.—  If,  on  a 
clear  evening  in  late  winter  or  in  spring, 
we  look,  just  at  the  close  of  twilight, 
at  the  part  of  the  horizon  at  which  the 
sun  has  set,  we  may  see  a  sort  of  au- 
rora of  faint  pearly  light,  of  a  half-oval 
figure,  with  its  base  resting  on  the  hori- 
zon, and  its  axis  coinciding  with  the 
ecliptic.  It  is  visible  for  nearly  90°  from 
the  sun,  growing  fainter  as  the  distance 
from  him  increases.  It  is  represented 
in  the  wood  -  cut,  Fig.  92.  It  is  a 
lens  -shaped  appendage  surrounding  the 
sun  and  lying  nearly  in  the  plane  of 
the  ecliptic.  It  is  called  the  Zodiacal 
Light. 


The  zodiacal  light  is  also  visible  in  the  autumn 
at  the  beginning  of  the  morning  twilight.  It  is 
seen  at  evening  on  or  near  March  2ist,  and  in  the 
morning  on  or  near  September  ist,  because  at  those 
seasons  and  hours  the  ecliptic  makes  its  greatest 
angle  with  the  horizon.  At  other  seasons,  when 
the  ecliptic  makes  a  smaller  angle,  the  zodiacal  light 
is  so  near  the  horizon  that  it  can  not  clearly  be  dis- 
tinguished, and  also  it  sets  before  the  sunlight  has 
entirely  faded.  People  who  live  within  the  trop- 
ics, where  the  air  is  very  clear,  can  sometimes  trace 
it  entirely  across  the  sky  from  east  to  west. 

It  obscures  very  small  stars  within  its  area  on 
the  heavens. 

Various  explanations  of  the  zodiacal  light  have 
been  suggested.  Some  observers  have  supposed 
that  it  might  be  an  extension  of  the  sun's  coro- 
na. The  most  common  opinion  now  attributes  it 
to  a  collection  of  very  minute  meteoroids  re- 
volving about  the  sun  nearly  in  the  plane  of  the 
ecliptic  and  reflecting  his  light.  When  the  zodia- 
cal light  is  visible,  it  marks  the  course  of  the  ecliptic 
very  clearly. 

FIG.  92. 


NOTE. — Biela's  comet  had  a  period  of  6%  years.  It  would  have  been 
due  in  1879,  but  neither  comet  nor  meteors  were  seen.  In  November,  1885, 
it  was  again  due.  While  this  book  is  going  through  the  press,  there  are  re- 
ceived full  accounts  from  observers  in  various  parts  of  the  world.  The  comet 
was  not  seen,  but  on  the  nights  of  November  25th,  26th,  27th,  there  was  a 


brilliant  meteor-shower  radiating  from  Andromeda  and  reaching  its  maxi- 
mum on  November  27th.  It  was  best  seen  in  the  eastern  part  of  Europe, 
though  visible  in  the  United  States.  It  is  now  regarded  as  nearly  certain 
that  Biela's  comet  has  broken  up  into  a  collection  of  meteoroids  revolving 
around  the  sun. 


THE  HEAVENS  OUTSIDE  OF   THE  SOLAR  SYSTEM. 


75 


PART    I  I  I. 

THE   HEAVENS   OUTSIDE   OF  THE  SOLAR  SYSTEM. 


CHAPTER    XI. 

218.  The  stars  are  usually  divided  into  fixed   stars 
and  planets.     The  latter  class  includes  the  stars  revolv- 
ing round  the  sun,  both  primaries  and  secondaries,  or 
moons.     All  other  stars  were  called  fixed  stars,  because 
they  showed  no  change  of  place  as  regards  each  other, 
though  they  all  apparently  revolve  round  the  earth. 

219.  Movements  of  the  Fixed  Stars. — The  ancient 
astronomers  at  Alexandria  ascertained  with  some  accu- 
racy the  relative  positions  of  the  chief  fixed  stars,  and,  in 
the  early  part  of  the  last  century,  Halley  compared  the 
records  left  by  them  with  the  results  of  observation,  and 
he  was  led  to  believe  that  the  so-called  fixed  stars  have 
changed  relative  place.     Owing  to  their  enormous  dis- 
tances from  us,  we  perceive  this  motion  very  slowly,  and 
so  it  was  only  detected  by  the  combined  work  of  men 
living  many  centuries  from  each  other.     Other  astron- 
omers began  to  investigate  the  subject.     Finally,  they 
came  to  the  conclusion  that  the  stars  examined  seemed 
all  to  be  moving  farther  from  a  point  in  Hercules,  and 
nearer  to  a  point  of  the  celestial  sphere  situated  exactly 
opposite  Hercules,  or  180°  from  him.     This  is  exactly 
the  appearance  which  would   be  produced  if  our  sun 
were  moving  toward  Hercules,  carrying  with  it  all  the 
bodies  dependent  on  it.     Thus  they  were  led  to  think  it 
probable  that  the  sun  has  a  real  motion.     This  motion, 
called  the  Secular  Motion  of  the  Sim,  appears  to  be  very 
slow,  only   because  it  is  measured   by   bodies  at  such 
enormous  distances  from  us. 

220.  Secular  Motion  of  Stars. — Besides  these  changes 
of  place,  which  could  be  explained  by  the  supposition 
that  we  ourselves,  with  the  solar  system,  are  in  motion, 
there  were  others  which  could  not  be  thus  accounted 
for,  and  they  led  to  the  belief  that  the  stars  have  a  very 
slow  real  motion  of  their  own,  which  is  called  the  Secular 
Motion  of  the  Stars.     The  slowness  of  the  motion  is  ap- 
parent, resulting  from  the  enormous  distance  of  the  stars. 
The  student  who  has  seen  a  train  of  cars  moving  at  a 
very  great  distance  remembers  that  they  seem  to  creep 
over  the  earth.    On  the  other  hand,  a  traveler  may  move 
rapidly  for  a  whole  day,  but,  if  he  measured  his  motion 
by  a  far-distant  mountain  only,  he  would  not  see  that  he 
had  changed  place. 

After  the  invention  of  the  spectroscope,  it  was  used 
in  studying  this  problem.    The  displacement  or  bending 


of  lines  toward  the  red  or  the  violet  end  of  the  spectrum 
indicates  motion  from  or  toward  the  observer.  The 
results  in  regard  to  the  motion  of  the  stars  confirm  the 
previous  conclusions. 

221.  Star-Drift. — It  is  found  that  stars  in  certain  parts 
of  the  heavens  have  motions  in  a  common  direction. 
Mr.  Proctor,  who  has  specially  studied  this  subject,  pro- 
poses for  this  motion  the  name  of  Star-Drift.  Thus  five 
stars  in  the  Great  Dipper  have  a  common  motion  in  the 
same  direction,  while  the  two  others  move  in  another 
direction  (see  Fig.  93).  This  must,  in  the  course  of  ages, 


FIG.  93. 


alter  the  figure  of  the  Great  Dipper.  The  spectroscope 
confirms  these  conclusions  by  showing  that  these  five 
stars  recede  from  us. 

222.  Motion  of  First-Magnitude  Stars. — It  will  inter- 
est students  to  know  that,  of  the  first-magnitude  stars, 
Sirius,  Regulus,  Betelguese,  and  Rigel  are  found  by  the 
spectroscope  to  be  receding  from  us,  while  Arcturus, 
Vega,  and   Pollux  approach   us.     The  rates  of  motion 
even  are  computed,  but  the  results  are  not  more  than  an 
approximation.     The  spectroscope,  the  student  must  re- 
member, only  tells  about  approach  toward  the  observer 
or  recession  from  him,  but  nothing  as  to  the  general  di- 
rection.    A  little  reflection  about  motions  of  persons  (or 
bodies)  whom  he  sees  on  earth  will  make  the  student 
understand  that  they  may  move  in  many  different  direc- 
tions, any  one  of  which  might  make  them  draw  near  us 
or  recede  from  us. 

Nothing,  therefore,  can  at  present  be  known,  from 
the  observed  facts,  in  regard  to  the  figure  of  the  sun's 
motion,  or  that  of  the  stars.  The  epithet  fixed  stars  is 
still  used  for  distinction. 

223.  Physical  Constitution  of  the  Stars. — The  spec- 
troscope shows  that  they  resemble  our  sun.     They  all 
show  the  dark  Fraunhofer  lines,  and  thus  it  is  evident  that 
the  luminous  spheres  are  enveloped  in  vapors  absorbing 
the  light  from  some  of  the  rays.     The  lines  show  that 


76 


ASTRONOMY  BY  OBSERVATION. 


they  contain  elements  known  to  us,  and  existing  in  our 
sun. 

The  spectra  of  different  stars  vary  somewhat.  They 
have  been  arranged  in  four  classes.  The  stars  vary  in 
color,  as  any  observer  may  see.  Thus,  Antares  and  Al- 
debaran  are  red  ;  Vega  Lyras,  Altair,  and  Spica  Virginis 
are  pale  blue;  Capella  and  Sirius  are  white;  Arcturus, 
Pollux,  and  Procyon  are  yellow.  The  colors  are  due  to 
vapors  in  their  atmospheres  cutting  off  part  of  the  light, 
and  the  colors  generally  mark  different  classes  of  stars. 
The  yellow  stars  are  more  like  our  sun. 

224.  Distances  of  the  Stars. — For  a  long  time  it  was 
thought  that  no  fixed  star  showed  any  apparent  change 
of  place,  due  to  the  earth's  change  of  position,  in  six 
months,  between  points   185,000,000  miles  apart.      But 
refined  and  accurate  instruments  and  methods  of  observa- 
tion have  shown  that  some  of  the  stars  exhibit  a  very 
small  displacement,  not  in  any  instance  amounting  to  i*. 
The  star  a  Centauri  has  a  pretty  well-ascertained  paral- 
lax, and  is  supposed  to  be  the  nearest  of  the  fixed  stars 
to  us.     But  the  distance  must  be  about  twenty  billions 
of  miles.     These  numbers  convey  little  idea  to  us,  and  it 
will  perhaps  give  a  more  definite  notion  to  the  student 
if  he  is  told  that  it  would  take  light  more  than  three 
years  to  reach  us  from  a  Centauri.     The  other  stars  are 
probably  nearly  all  much  farther  from  us. 

225.  Magnitude  of  the  Stars. — None  of  the  fixed  stars 
show  any  disk  even  when  examined  with  the  most  pow- 
erful telescopes.     This  is  one  way  in 

which  observers  with  the  telescope 
readily  distinguish  planets  from  stars. 
The  telescope  aids  us  in  observing  a 
star  merely  by  its  power  of  collecting 
light.  It  does  not  magnify  the  stars, 
as  they  are  too  far  off.  Therefore,  we 
know  nothing  of  their  magnitudes. 

226.  Numbers  of  the  Stars.  —  Ob- 
servers with  the  naked  eye  see  about 
five   thousand  stars.     With  the  great 
telescopes  of  modern  times  they  see 
many   millions,    but   no    estimate    has 
been  made  approaching  exactness. 

227.  Double  and  Multiple  Stars.— 
Some   stars   which   are  single  to  the 
naked  eye  are  resolved  by  a  telescope 
into  two  or  three  or  sometimes  more 
stars.     In  some  cases,  this  is  due  to  the 
fact  that  stars  not  near  together  are  on 
the  same  line  of  vision,  and   in   such 
cases    they  are  called   optically    double 
stars.      But   in    many   instances,  it    is 
found  that  these  stars  are  connected 

/-—.  . 


FIG.  94. 


Orbit  of  a  double  Star, 


FIG.  95. 


by   revolving  around  a  common 

center ;  and  in  that  case,  they  are 

called  physically  double  stars.     The 

motion,  and  even    the  period    of 

some  stars  is  clearly  determined,  as 

in  £  of  the  Great  Bear,  which  com- 
pletes a  revolution  in  sixty  years. 

e  Lyras   can    be    resolved  into    a 

double  star  by  a  good  opera-glass 

(young  people  with  good  eyes  see 

it  double  without  a  glass),  and  a  powerful  telescope 

shows  that  each  of  these  stars 
is  double.  (See  Figs.  94  and 

95-) 

Double  and  multiple  stars 
are  very  often  of  different  col- 
ors, and  sometimes  of  comple- 
mentary colors. 

228.    Variable   Stars.— This 
name  is  applied  to  stars  which 
have      periodical      variations. 
There  are  many  of  these.     The 
most    noted   of   them   are    the 
stars  Algol  or  ft  Persei ;  Mira 
in  Cetus,  or   the    Whale ;   and 
Eta,  or  i),  in  the  ship  Argo. 
The  changes  of  Algol,  or  ft  Persei,  are  of  such  short 
period  that  any  observer  may  detect  them  if  he  knows 

FIG.  96. 


The  double-double  Star  in  the 
Constellation  Lyra.  i.  As 
seen  in  an  opera-glass.  2. 
As  seen  in  a  small  telescope. 
j.  As  seen  in  a  telescope  of 
great  power. 


'J  he  Pleiades  in  a  large  Telescope. 
7 U*~.l*.4      v>£*"£    -*«-- 


-.?. 


' 


•iL^l 


y 


THE  HEAVENS  OUTSIDE  OF   THE  SOLAR   SYSTEM. 


77 


when  to  look  for  them.  Algol  occupies  seven  hours 
out  of  every  sixty-nine  in  making  a  gradual  change  in 
luster  or  brilliancy.  Algol  is  usually  a  faint  star  of  the 
second  magnitude,  but  during  twenty  minutes  at  the 
middle  of  the  seven  hours,  it  becomes  a  star  of  the  fourth 
magnitude.  The  change  is  very  gradual.  In  order  to 
remark  it,  it  is  necessary  to  compare  Algol,  before  the 
beginning  of  the  variation,  with  some  other  star  of  the 
same  size.  This  gives  a  standard  by  which  to  observe 
the  change.  The  exact  period  in  which  the  variations 
of  Algol  are  completed  is  two  days,  twenty  hours,  forty- 
nine  minutes. 

Mira  in  the  Whale,  or  o  Ceti,  as  the  star  is  called  by 
astronomers,  is  a  star  whose  variations  can  be  seen  with 
the  naked  eye.  It  has  a  period  of  nearly  a  year.  From 
a  star  of  the  second  magnitude  it  becomes  invisible.  Its 
variations  are  not  altogether  regular. 

Eta,  in  the  ship  Argo,  or  the  star  rj  Argus,  is  vari- 
able. This  is  a  star  of  the  southern  hemisphere.  It  was 
seen  by  Sir  John  Herschel  brighter  than  any  other  star 
except  Sirius,  and  he  says  it  then  began  to  decrease  and 
passed  slowly  out  of  sight. 

229.  New  Stars. — There  are  some  instances  of  stars 
which  have  suddenly  appeared  and   shone   for  a  time 
with  great  brilliancy,  disappearing  afterward.     In  the 
year  1572,  such   a   star  appeared   in   the   constellation 
Cassiopeia,  and  was  described  by  the  astronomer  Tycho 
Brahe.     It  outshone   Jupiter  and   Venus,  and  could  be 
seen  at  noon.     After  six  months,  it  disappeared,  and  has 
not  been  heard  of  since.     It  underwent  several  changes 
of  color  while  visible.     In   1604,  a  new  star  of  the  first 
magnitude  appeared  in  the  constellation  Ophiuchus.     It 
was  visible    more   than  a  year,  and   then  disappeared. 
This  is  sometimes  called  Kepler's  star,  because  this  as- 
tronomer observed  and  recorded  its  changes  and  appear- 
ance.    In  1866  a  telescopic  star  in  Corona  Borealis  sud- 
denly increased  to  the  second  magnitude  and  afterward 
faded.     It  was  examined  with  the  spectroscope  and  gave 
evidence  of  a  conflagration  in  which  hydrogen  was  the 
chief  agent.     The  red  flames  of  the  sun's  chromosphere 
are  due  to  hydrogen,  and  it  is  supposed  that  this  star 
showed  phenomena  like  the  red  prominences,  only  on  a 
much  larger  scale.* 

230.  Star-Clusters. — The  Pleiades  are   a    cluster  of 
stars.     The  ordinary  observer  sees  six  stars  ;  good  eyes 
sometimes  detect  seven ;   a  small  telescope   brings  out 
many  more ;  and  one  of  the  large  telescopes  shows  over 
five  hundred.     (See  Figs.  96,  97.)     The  telescope  shows 
fine  globular  clusters  in  Hercules,  in  Aquarius,  and  in 
Toucan,  in  the  southern  hemisphere.     One  of  the  finest 

•  See  note  at  the  end  of  Chapter  XI. 


is  in  Centaurus.  In  the  sword-handle  of  Perseus,  there 
is  a  star  which  appears  hazy  to  the  naked  eye,  but  a  tele- 
scope of  moderate  power  shows  that  it  is  a  very  brilliant 
cluster. 

231.  The  Galaxy,  or  Milky- Way. — This  has  been  de- 
scribed in  Chapter  I.  When  examined  through  the  tele- 
scope, it  is  seen  to  be  composed  of  very  small  stars 
whose  combined  light  creates  the  milky  appearance.  It 

FIG.  97. 


Star-Clusters,     i.  In  Libra.     2.  /«  Hercules,    j.  In  Capricornus.     4.  In  Ser- 
pens.    j.  In  Aquarius.     6.  In  Gemini. 

contains  many  clusters  of  stars.  The  numbers  of  the 
fixed  stars  in  general  increase  in  the  direction  of  the 
Milky- Way  and  in  it.  This  is  more  evident  when  we 
use  a  telescope,  for  the  telescopic  stars  greatly  out-num- 
ber the  others. 

232.  Nebulae. — A  nebula  looks  like  a  patch  of  cloudy 
light.  There  are  two  classes  of  nebulae ;  those  whose 
light  is  shown  to  be  due  to  a  great  collection  of  telescop- 
ic stars,  and  those  which  no  power  of  the  telescope  yet 


ASTRONOMY  BY  OBSERVATION. 


FIG.  98. 


FIG.  99. 


Planetary  Nebula  in 
Ursa  Major. 


Elliptical  Nebula  near 
•y  Andromeda. 


FIG.  100. 


used  by  us  has  resolved  into  stars.  Of  the  latter,  many 
have  been  shown  by  the  spectroscope  to  be  masses  of 
glowing  gas. 

There  is  a  remarkable  nebula  in  the  sword-handle  of 

Orion.  It  can 
be  seen  by  the 
naked  eye  sur- 
rounding the 
middle  of  the 
three  stars  in 
the  handle.  It 
was  examined 
by  Professors 
Secchi  and 
Huggins  with 
the  spectroscope,  and  they  found  lines  leading  them  to 
believe  it  composed  of  glowing  gas.  Another  remarka- 

FlG.  IOI. 


Ring-Nebula  in  Lyra. 


FlG.  IO2. 


ble  nebula  which  can  be  seen  with  the  naked  eye  is  the 
great  nebula  of  Andromeda.  The  triangle  in  Cancer 
contains  a  nebula  which  can  be  seen 
without  the  telescope.  It  is  called 
Prassepe,  or  the  Bee-hive  Nebula.  Great 
numbers  of  nebulae  are  seen  with  the 
telescope,  but  they  are  most  numerous 
at  a  distance  from  the  Milky  -  Way. 
Some,  as  seen  in  Fig.  98,  are  round  in 
appearance  like  a  planet,  and  are  there- 
fore called  Planetary  Nebulae.  Other 
nebulae  are  elliptical  (Fig.  99),  while  still  others  are  ring- 
shaped  (Fig.  100).  Some  are  oval  (Fig.  101).  Somecon- 


Nebulous    Star, 
i  Orionis. 


FIG.  103. 


Crab  Nebula  in  Taurus. 


sist  of  a  hazy  circle  surrounding  a  star,  which  is  there- 
fore called  a  nebulous  star  (Fig.   102).     Besides  these, 

FIG.  104. 


FIG.  105. 


there  are  nebulae  of  many  irregular  shapes  (Fig.  103). 
Some  are  spiral,  and  look  as  if  they  might  be  in  rotation 
around  some  central 
point.  (See  Figs. 
104  and  105.)  Fig. 
too  is  a  representa- 
tion of  the  nebula  of 
Orion. 

233 .  The  Ma- 
gellanic  Clouds. — 
Travelers  in  the 
southern  hemi- 

sphere of  the  earth 
long  ago  brought 
accounts  of  two 
large  masses  of 
cloudy  light  which 
they  saw  near  the 
south  pole  of  the 

heavens.      They  are  Spiral  Nebula  in  Canes  Venatici, 


THE  HEAVENS  OUTSIDE  OF   THE  SOLAR   SYSTEM. 


79 


called  the  Magellanic  Clouds,  and  they  are  further  dis- 
tinguished from  each  other  as  Nubecula  Major  and  Nu- 

becula  Minor. 

FIG.  106.  XT    i          i     » * 

Nubecula  Ma- 

jorisofcoursethe 
larger,  and  it  is 
so  bright  that  it 
is  not  obscured 
by  the  full  moon, 
which  causes  Nu- 
becula Minor  to 
become  invisi- 
ble. When  seen 
through  a  power- 
ful telescope,they 
are  both  resolved 
into  stars  and  sep- 
arate nebula?.  Of  the  latter,  Herschel  counted  278,  be- 
sides more  than  fifty  outlying  nebulae.  (See  Fig.  107.) 

FIG.  107. 


' 


Great  \ebula  of  Orion. 


Part  of  the  Nubecula  Major. 

234.  The  Nebular  Theory. — A  great  many  astrono- 
mers believe  that  suns  and  planets  are  formed  from  neb- 
ulae.* The  nebulous  matter  is  supposed  to  be  very  hot 
and  rotating  rapidly.  It  gradually  condenses  and  flat- 
tens into  a  rotating  disk.  This  throws  off  rings  which 
revolve  and  finally  condense  into  planets,  the  central  and 

*  Many  of  those  who  believe  this  theory  also  believe  in  a  Creator,  and  in 
the  Christian  religion. 


large  mass  constituting  a  sun.  The  facts  which  they 
adduce  in  support  of  this  theory  are :  the  evidences  of 
the  earth's  former  fused  condition  ;  the  flattening  at  the 
poles  seen  both  in  earth  and  planets ;  the  fact  that  plan- 
ets and  moons  (with  two  exceptions)  all  revolve  from 
west  to  east,  or  in  the  same  direction  with  each  other ; 
the  rings  of  Saturn ;  the  partially  fused  and  vaporous 
condition  of  the  larger  planets,  which  would  cool  more 
slowly ;  and  the  spiral  form  common  among  the  neb- 
ulas. 

The  opponents  of  this  theory  consider  the  fact  that 
the  moons  of  Uranus  and  Neptune  revolve  from  east  to 
west  a  serious  objection  to  it,  and  also  the  fact  that  one 
of  the  moons  of  Mars  revolves  around  Mars,  in  a  shorter 
time  than  Mars  revolves  on  his  axis. 

235.  Southern  Circumpolar  Constellations. — To  peo- 
ple in  the  United  States,  these  are  in  the  Circle  of  Per- 
petual Disparition,  so  that  the  student  can  not  see  them 
in  nature,  but  he  should  know  something  of  them.  An 
account  of  them  is  placed  here,  because  they  may  be 
overlooked  in  the  Description  of  Constellations  in  the 
Appendix,  which  is  intended  for  reference  only. 

The  chief  groups  are:  Ara,  the  Altar;  Crux,  the 
Cross ;  Dorado,  the  Sword-Fish ;  Hydrus,  the  Water- 
Snake ;  Pavo,  the  Peacock ;  the  Southern  Triangle,  and 
Toucanus,  the  Toucan.  Three,  viz. :  Phoenix ;  Grus,  the 
Crane ;  and  Centaurus,  the  Centaur,  can  be  partially  seen 
in  the  Southern  States.  The  Southern  Cross  contains 
one  first-,  three  second-magnitude  stars.  It  is  the  glory 
of  the  Southern  skies.  There  are  six  first-magnitude 
stars  around  the  southern  pole.  One  of  these,  Canopus, 
can  be  seen  in  Tennessee.  It  presents  a  fine  appearance 
in  Georgia,  and  in  the  clear  skies  of  Florida  it  is  an  ob- 
ject of  much  interest  to  visitors  acquainted  with  astron- 
omy. It  is  south  of  Sirius.  Three  of  these,  Bungula 
and  Agena  in  Centaurus  and  Acherner,  can  be  seen  in 
the  southern  parts  of  Louisiana,  Texas,  and  in  Florida. 

NOTE. — In  the  summer  of  1885,  a  new  star  appeared  in  the  nebula  of 
Andromeda.  It  could  not  be  seen  except  with  the  telescope.  It  increased 
in  luster  from  its  appearance  in  August  until  September,  when  it  had  at- 
tained the  size  of  a  star  of  7^2  magnitude.  It  then  began  to  decrease,  and 
by  September  2Oth  was  of  the  gth  magnitude.  This  phenomenon  is  sup- 
posed to  be  due  to  some  sudden  evolution  of  burning  gas  in  a  star  previously 
too  small  to  be  seen  even  with  the  most  powerful  telescope. 


NOTE. — Since  this  book  was  written,  the  leading  astronomers  of  the  world 
have  formed  a  plan  for  combining  to  photograph  the  whole  heavens,  dividing 
it  into  portions  distributed  among  themselves.  Celestial  photography  has 
produced  some  maps  of  unexampled  accuracy,  which  also  show  stars  not 


made  visible  by  the  telescope  alone.  The  execution  of  this  project,  which 
will  take  several  years,  will  probably  much  enlarge  our  knowledge  of  the 
heavens.  In  April,  1887,  the  leading  astronomers  met  in  Paris  to  discuss 
this  project. 


APPENDIX   A. 


DESCRIPTION     OF     CONSTELLATIONS. 

ALPHABETICALLY  ARRANGED. 
(These  are  intended  merely  for  reference,  to  aid  students  in  learning  the  constellations  from  nature.) 


f      I. — 1st  m.,  2d  m.,  etc.,  are  applied  to  stars,  to  designate  first  magnitude,  second  magnitude,  etc. 

ABBREVIATIONS,  -j    II.— E.,  W.,  N.,  S.,  N.  E.,  S.  E.,  N.  W.,  S.  W.,  are  used  to  designate  the  various  points  of  the  compass. 
(  III. — Z.  C.     These  letters  are  affixed  to  all  Constellations  of  the  Zodiac. 


ANDROMEDA. 


This  constellation  is  best  learned 
after  Pegasus.  The  N.  E.  star  in 
the  figure  called  the  Great  Square  of  Pegasus,  belongs  to  An- 
dromeda. This  and  two  other  2d  m.  stars  extend  N.  E.  from 
Pegasus,  in  a  line  not  quite  straight.  There  are,  also,  two 
3d  m.  stars  on  the  map.  The  Great  Nebula  of  Andromeda  is 
one  of  the  most  remarkable  in  the  heavens. 

A  DITTO      ~7    r*  Aries  is  best  learned  after  Androm- 

/llxlll.0.  Z.      (     .  j  -p.*  rrt  l-r^-t 

eda,  Pisces,  or  Taurus.     There  is  a 

small  irregular  triangle,  containing  one  zd  m.,  two  3d  m.  stars. 
This  must  not  be  confused  with  another  triangle,  which  is  the 
constellation  Triangula.  Triangula  is  a  slender,  nearly  isosceles 
triangle,  which  contains  no  2d  m.  star.  Both  are  S.  E.  from 
Andromeda.  All  other  stars  of  Aries,  except  the  three  in  the 
triangle,  are  very  faint.  The  ecliptic  runs  a  little  more  than  8° 
south  of  the  triangle,  which,  therefore,  is  not  in  the  Zodiac, 
though  part  of  the  constellation  is  there.  No  star  visible  to  the 
unaided  eye  marks  the  ecliptic. 

The  Water-Bearer.  Also,  Fomalhaut 
of  Piscis  Australis.  Aquarius  con- 
tains a  small  Y,  formed  of  4th  m.  stars,  which  is  quite  distinct 
and  easy  to  find,  since  it  is  not  far  S.  W.  from  the  Great  Square 
of  Pegasus.  There  is  also  a  figure  which,  in  shape,  resembles 
somewhat  the  continent  of  South  America.  S.  A.  has  3d  m. 
stars  at  the  angles,  but  the  others  are  faint.  There  is  risk  of 
getting  students  to  call  it  South  America,  by  which  name  it  is 
of  course  not  known  to  astronomers ;  but  when  attention  is 
called  to  the  resemblance,  it  makes  the  constellation  much 
easier  to  find.  A  line  from  the  little  Y,  through  S.  A.,  reaches 
the  ist  m.  star,  Fomalhaut,  in  the  eye  of  the  Southern  Fish. 
Fomalhaut  is  farther  south  than  any  other  ist  m.  star  except 
Canopus,  which  is  not  seen  north  of  Tennessee.  To  the  east 
of  S.  A.  there  are  a  number  of  small  stars  in  little  clusters  of 
twos  and  threes.  Aquarius  is  a  man  pouring  water  from  a 
cup.  The  little  Y  is  on  the  cup,  and  the  water  is  pouring  from 
it.  The  little  clusters  of  twos  and  threes  are  in  the  stream,  and 
so  are  Fomalhaut  and  the  Fish.  The  ecliptic  crosses  S.  A.,  and 


AQUARIUS.    Z.  C. 


east  of  S.  A.  are  two  4th  m.  stars  joined  together  by  a  line  on 
the  map,  which  mark  its  course.  The  two  southwest  stars  in 
S.  A.  belong  to  the  constellation  Capricornus.  This  should  be 
impressed  upon  students  after  they  find  S.  A.  It  is  almost  im- 
possible to  have  Aquarius  learned  without  using  the  distin- 
guishable figure  S.  A.,  and  with  care  there  will  be  no  confusion. 
The  author  has  taught  Aquarius  to  many  children  aged  twelve, 
and  always  without  going  out  with  them  at  night. 

AHTTTT  A  Or  *^e   ^aS^e-     Aquila  is   easily  distin- 

^  '  guished  by  three  stars  in  a  short  line, 

lying  nearly  across  the  Milky-Way.  One  is  the  ist  m.  star 
Altair.  Aquila  comes  in  summer,  when  the  brightest  part  of 
the  Milky- Way  is  visible.  The  figure  of  A.  on  the  map  contains 
3d  M.  stars,  and  is  easily  recognized.  It  lies  in  the  Milky-Way, 
nearly  overhead  when  on  the  meridian.  One  of  the  stars,  Eta, 
is  on  the  equinoctial.  A.  can  be  learned  by  its  position  in  the 
Milky- Way,  without  knowing  any  adjacent  constellation  ;  or  A. 
may  be  learned  after  Sagittarius.  Altair  can  also  be  known  by 
being  the  brightest  star  of  the  Milky- Way  as  seen  in  summer. 

or  the  Wagoner.     This  can  be  learned 

AURIGA,  after  Orion;  PerseuS)  or  Gemini.     It  is 

very  conspicuous,  and  easily  found.  It  is  an  irregular  five-sided 
figure,  containing  one  ist  m.  star,  Capella,  and  two  2d  m.  stars. 
There  is  a  small,  slender  triangle,  called  "  The  Kids,"  which  is 
easy  to  find. 

The  Ship  Argo.  This  may  be  learned 
after  Canis  Major,  from  which  it  is  S.  E. 
There  are  a  number  of  bright  stars  on  the  southern  horizon, 
but  they  are  not  combined  into  any  figure  on  the  map.  They 
are  easily  found.  One  2d  m.  star,  Naos,  is  clearly  seen ;  and 
in  the  Southern  States,  another  2d  m.  can  be  recognized  on 
clear  nights. 

Bootes  can  be  easily  found  after  the 
Great  Dipper  is  known,  from  which  it 
is  S.  E.  In  early  spring,  when  the  Great  Dipper  is  in  the  east, 
a  brilliant  ist  m.  star  can  be  seen  in  the  northeast,  long  before 
the  whole  constellation  Bootes  can  be  found.  This  star  is 


ARGO   NAVIS. 


BOOTES. 


APPENDIX  A.— DESCRIPTION  OF  CONSTELLATIONS. 


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82 


ASTRONOMY  BY  OBSERVATION. 


Arcturus,  and  in  looking  for  Bootes  it  is  best  to  find  Arcturus 
first.  Three  stars  are  in  line  with  Arcturus,  and  two  of  these,  with 
others,  form  a  figure  resembling  a  kite.  There  is  also  a  small 
triangle  S.  W.  of  A.,  with  one  3d  m.  star.  Also,  S.  E.  from  A., 
there  are  three  stars  nearly  in  line,  of  which  one  is  3d  m. 

/-AXT/-C-D       -7  Cancer  is  a  very  inconspicuous 

i^AlNv^llK.      i,.  C.  .   ,,    ..        ,  ,    ..„- 

constellation,  but  not  at  all  dim- 
cult  to  find  after  Gemini  or  Leo  are  well  known.  It  is  between 
them,  a  little  S.  E.  from  Gemini,  a  little  N.  E.  from  the  Sickle 
in  Leo.  There  is  a  small  triangle  of  4th  and  5th  m.  stars,  made 
more  evident  by  having  in  its  center  a  patch  of  cloudy  light* 
which  is  Praesepe,  or  the  Bee-hive  nebula.  In  the  telescope,  it 
is  a  cluster  of  stars.  The  triangle  is  important,  because  one  of 
the  most  southern  stars  is  on  the  ecliptic. 

#     CANIS  MAJOR.  The   Greater    Dog    sometimes 

called  Sinus,  from  the  name  of 

its  ist  m.  star  Sirius,  which  is  the  brightest  of  all  the  fixed 
stars.  Sirius  is  sometimes  called  the  "dog-star,"  and  the  dog- 
days  of  summer  owe  their  name  to  the  fact  that  Sirius  is  on  the 
meridian  above  us  at  noon,  during  the  dog-days.  Of  course, 
we  can  not  see  him  there,  but  his  position  is  known  by  calcu- 
lation. The  Greater  Dog  is  best  learned  after  Orion,  from 
which  it  is  S.  E.  After  drawing,  there  is  no  possible  chance  to 
mistake  it. 

When  Sirius  is  south  of  us,  there  can  be  seen  in  Florida,  and 
States  in  the  latitude  of  Georgia,  a  fine  ist  m.  star,  Canopus, 
just  above  the  southern  horizon.  It  is  so  far  south  that  it  makes 
but  a  small  arc  in  crossing  the  sky,  and  is  above  the  horizon 
but  a  few  hours.  As  it  is  so  near  the  horizon,  it  is  liable  to  be 
blotted  out  by  smoke  or  fog  except  on  very  clear  evenings.  But 
the  observer  in  the  States  mentioned  should  look  for  it.  It  can 
be  seen  in  the  latitude  of  Tennessee  by  a  person  familiar  with 
it ;  but  it  is  very  near  the  horizon,  and  its  luster  is  somewhat 
obscured. 

/-AXTTC-   TV/TTVT^-I  The  Lesser  Dog.    This  can  best 

CANIS  MINOR.  j    r      /- 

be  learned  after  Gemini,  Orion, 

or  Leo.  It  is  S.  or  a  little  S.  E.  from  Gemini,  nearly  E.  from 
the  northern  part  of  Orion,  and  S.  W.  from  Leo.  There  are 
two  stars  on  the  map;  one  a  ist  m.  star,  Procyon,  the  other 
3d  m.  Procyon  and  the  3d  m.  star  are  about  as  far  from  each 
other  as  Castor  and  Pollux,  and,  if  they  were  more  nearly  equal 
in  magnitude,  would  look  a  good  deal  like  Castor  and  Pollux. 

The  Hunting  Dogs.  These  form 
a  very  inconspicuous  constella- 
tion between  the  Great  Dipper  and  Bootes,  and  contain  only 
a  single  3d  m.  star  midway  between  the  two,  which  is  not 
at  all  difficult  to  find.  They  are  put  in  this  "  Description " 
because  the  constellation  shows  in  the  telescope  a  very  remark- 
able spiral  nebula,  mentioned  in  another  part  of  this  book. 

CAPRICORNUS.    Z.C.    ™S  is  best  !earned  ""^L8* 

gittanus   or  Aquarius.      There 

are  two  3d  m.  stars  near  each  other,  and  forming  a  short  line 
nearly  at  right  angles  with  the  line  of  the  ecliptic.  It  is  mid- 
way between  Sagittarius  and  Aquarius,  and  is  easily  found. 
The  ecliptic  is  a  little  farther  from  the  most  southern  star  than 


CANES  VENATICI. 


AccTr»r>nT  A 
AoolL/i  JtLl  A, 


they  are  from  each  other.  In  the  figure  in  Aquarius  resembling 
South  America,  the  two  3d  m.  stars  in  the  S.  W.  angle  belong 
to  Capricornus. 

Cassiopeia  is  best  learned  after  the 
two  Dippers.  If  a  line  is  drawn 
connecting  the  stars  in  the  handle-ends  of  the  two  Dip- 
pers, and  it  is  prolonged  on  the  side  of  Polaris,  it  will  pass 
through  part  of  the  figure  of  Cassiopeia,  given  on  our  map.  In 
late  summer  and  early  autumn  Cassiopeia  is  to  the  right  of  the 
northern  sky  at  dark  ;  in  late  autumn  and  early  winter  she  is 
on  or  near  the  meridian  at  dark  ;  in  late  winter  and  early  spring 
she  is  to  the  left  of  the  northern  sky  at  dark.  The  figure  made 
by  the  bright  stars  and  one  faint  one  resembles  a  chair,  and  is 
often  called  Cassiopeia's  Chair.  The  faint  star  is  in  the  front 
of  the  seat  of  the  chair.  Without  this  star,  the  figure  of  Cassio- 
peia somewhat  resembles  a  very  irregular  W. 

rprM_ATTRTT_         The  Centaur.     Centaurus  is  only  par- 
f\UKUS.       tiallv  seen  in  the  United  StateSj  and 

is  on  the  southern  horizon,  S.  W.  of  Scorpio.  Its  most  brilliant 
stars  (ist  m.)  are  below  the  horizon  in  the  U.  S.  There  are, 
however,  some  fine  3d  m.  stars.  There  is  a  figure  of  a  large  Y ; 
a  very  small  and  not  bright  triangle  above  it;  a  slender,  nearly 
isosceles  triangle  east  of  the  Y ;  and,  to  the  west  of  the  other 
figures,  two  stars  near  the  horizon.  Centaurus  is  visible  in  the 
late  spring. 

Cepheus  is  an  inconspicuous  con- 
stellation near  the  pole.  The  figure 
resembles  an  irregular  K.  It  lies  between  Cassiopeia,  the 
head  of  Draco,  and  the  Little  Dipper,  and  is  best  learned  in 
summer,  late  spring,  or  early  autumn.  It  is  N.  of  the  Cross 
in  Cygnus. 

or   the  Whale.     This  constellation   is 


CEPHEUS. 


/^T7  TT  T  T  C 


best  learned  after  Pegasus,  Aquarius, 


and  Pisces.  It  consists,  as  can  be  seen  on  the  map,  of  a 
quadrilateral  of  3d  m.  stars,  E.  of  Aquarius,  and  easily  dis- 
tinguished. One  of  the  stars  of  the  quadrilateral  has  another 
near  it.  N.  E.  from  this  quadrilateral  there  is  a  triangle 
containing  a  2d  m.  star.  On  the  map,  the  triangle  and  the 
quadrilateral  are  joined,  forming  a  figure  resembling  the  mantis 
insect,  popularly  called  the  "devil-horse."  E.  of  the  quadri- 
lateral, and  between  it  and  Aquarius,  are  two  stars,  one  id  m., 
one  3d  m.,  joined  on  the  map  by  dotted  lines.  E.  of  the  quad- 
rilateral there  is  a  small  square  composed  of  3d  and  4th  m. 
stars,  from  which  a  chain  or  stream  of  stars  runs  south,  forming 
part  of  the  constellation  Eridanus.  Cetus  contains  the  remark- 
able variable  star  Mira,  but  it  is  half  the  time  invisible.  It 
forms  a  triangle  with  the  N.  E.  star  of  the  quadrilateral  and 
the  N.  W.  star  of  the  small  square. 

or  the  Dove.     S.  of  Lepus  and  Orion 
COLUMBA,  there  is  a  small  duster  of  stars  con. 

taining  one  zd  m.  and  one  3d  m.  star.  This  is  Columba.  The 
2d  m.  star  is  called  Phaet. 


COMA  BERENICES. 

can  best  be  learned  after  Bootes  or  Virgo.     It  lies  N.  from 


APPENDIX  A.— DESCRIPTION  OF  CONSTELLATIONS. 


CORVUS. 


CYGNUS, 


DELPHINUS, 


Virgo,  and  W.  from  Bootes.  It  lies  directly  S.  from  the  Great 
Dipper,  but  can  be  seen  only  when  the  Dipper  is  above  the  pole. 
The  Crow.  The  Crow  is  standing  on 
the  Water-Serpent,  Hydra,  and  is  best 
learned  with  it.  It  is  an  irregular  quadrilateral,  and  the  star  at 
two  of  the  angles  has  another  near  it.  It  is  very  easily  distin- 
guished, and  is  just  S.  of  Virgo.  The  solstitial  colure  passes 
near  it. 

'  CORONA  BOREALIS.    The    Northern    Crown       This 

constellation    is    best    learned 

after  Bootes,  from  which  it  is  E.  ;  or  Ophiuchus,  from  which 
it  is  W.  It  is  easily  distinguished  by  a  figure  resembling  a 
semicircle,  and  by  the  2d  m.  star  Gemma. 

or  the  Swan.  Cygnus  is  visible  in  sum- 
mer and  autumn,  and  is  easily  found, 
because  it  is  in  the  part  of  the  Milky- Way  then  visible.  It  may 
also  be  learned  after  Pegasus,  Hercujes,  or  Aquila.  It  is  in  the 
part  of  the  Milky- Way  N.  E.  from  Aquila.  The  figure  of  a  cross 
is  so  evident  that  it  is  easily  identified.  The  upper  or  northern 
part  of  the  cross  is  composed  of  bright  stars,  and  the  arms  are 
in  a  line  across  the  Milky-Way.  The  star  at  top  of  the  cross, 
D_ene_b,  is  a  very  bright  2d  m.  star.  The  upper  part  of  the  cross 
should  be  found  first. 

or  the  Dolphin.  Delphinus  may  be 
learned  after  Cygnus,  from  which  it  is 
S.  E. ;  or  after  Aquila,  from  which  it  is  N.  E. ;  or  after  Pegasus, 
from  which  it  is  nearly  W.  It  is  easily  distinguished  in  sum- 
mer and  autumn,  because  it  is  very  near  the  middle  point  of  the 
part  of  the  Milky- Way  then  above  the  horizon,  and  on  its  east- 
ern side.  It  consists  of  a  small  diamond-shaped  figure,  with 
another  star.  All  except  one  of  these  stars  are  3d  m.  The 
diamond  or  lozenge  figure  is  popularly  called  Job's  Coffin. 

or   the    Dragon.      Draco    can    best   be 
'  learned  in  late  spring,  summer,  or  early 

autumn.  It  should  be  learned  after  the  student  knows  the 
Dippers  and  the  ist  m.  star  Vega  in  Lyra.  The  two  Dip- 
pers should  be  drawn  first,  and  Draco  added  afterward.  In 
looking  for  it,  the  student  should  be  directed  to  look  first  for 
the  head.  A  straight  line  from  Vega  to  the  bowl  of  the  Little 
Dipper  passes  through  the  head  of  Draco.  It  is  a  very  small, 
irregular  quadrilateral,  containing  two  2d  m.  stars  which  can 
not  fail  to  be  recognized.  The  two  other  stars  of  the  quadri- 
lateral are  fainter.  One  is  3d  m.,  one  4th  m.  After  finding 
the  head,  the  student  must  look  for  the  tail.  A  chain  of  stars 
forming  it  surrounds  the  Little  Dipper,  passing  between  it  and 
the  Great  Dipper ;  and,  after  making  more  than  half  the  circuit, 
it  makes  a  turn  back  to  join  the  head.  One  of  these  stars  is  of 
interest,  because  the  pole  of  the  heavens  was  once  very  near  it. 
It  is  called  a  of  Draco,  and  it  lies  in  a  straight  line  between  £, 
a  star  in  the  bend  of  the  handle  of  the  Great  Dipper,  and  y. 
one  of  the  two  stars  called  "  Guardians  of  the  Pole,"  which  lies 
in  the  bottom  of  the  bowl  of  the  Little  Dipper. 

or  the  River  Po.    This  constellation  lies 
between  Cetus  and  Orion.     There  are 
two  irregular  chains  of  stars,  of  which  one  extends  from  the 


HERCULES. 


HYDRA. 


triangle  in  Cetus  to  Rigel  in  Orion  (a  ist  m.  star).  It  contains 
a  2d  m.  star.  The  other  chain  runs  from  the  little  square  in 
Cetus  to  the  southern  horizon. 

>V  or    the    Twins.     This    is   best    learned 

UEMINI,   Z.  C.,      after  Orion  or  Leo      The  twQ  blightest 

stars  are  called  Castor  and  Pollux.  Pollux  is  called  ist  m., 
Castor  2d  m.,  but  it  takes  close  observation  to  see  any  differ- 
ence. There  are  three  stars  in  Gemini  lying  nearly  on  the 
ecliptic,  and  one  of  these,  the  extreme  western  star  of  the  map, 
is  a  little  E.  of  the  point  where  the  sun  is  situated  on  June  22d. 
The  point  is  called  the  Summer  Solstice.  The  student  should 
mark  this  point  S.  S.,  in  drawing  the  constellation. 

Hercules  is  best  learned  after  Bootes 
Ophiuchus,  or  Draco.  After  drawing 
Hercules  and  Ophiuchus  separately,  and  finding  them,  they 
should  be  drawn  together,  because  the  figures  run  together. 
Hercules  contains  no  stars  brighter  than  3d  m.  The  solstitial 
colure  runs  just  E.  of  Hercules. 

The  Water-Snake.  Hydra  is  best  found 
in  April  or  May,  after  Virgo,  Leo,  Can- 
cer, and  Libra  are  known,  for  it  lies  S.  of  them  all.  It  is 
a  long  constellation,  extending  N.  W.  and  S.  E.  It  is  best  to 
find  the  head  first.  That  lies  just  S.  of  Cancer,  and,  though 
composed  only  of  4th  and  5th  m.  stars,  is  perfectly  distinct  and 
easily  found.  It  looks  a  good  deal  like  the  nodding  bud  of  a 
fuchsia-blossom.  After  learning  the  head,  the  student  will  do 
well  to  find  the  2d  m.  star,  Al  Fard,  or  Cor  Hydra,  which  is  S. 
of  Leo.  Then  he  finds  Corvus,  a  separate  constellation  (see 
CORVUS),  S.  of  Virgo,  but  one  which  should  be  drawn  with 
Hydra.  W.  of  Corvus,  three  3d  m.  stars  extend  in  an  almost 
E.-W.  line  toward  Libra.  Between  Corvus  and  Cor  Hydra 
there  are  a  good  many  very  small  stars,  but  it  is  not  important 
to  distinguish  them  ;  and  it  is  well  to  let  the  student  represent 
them  in  his  drawing  by  a  few  flourishes.  Part  of  them  consti- 
tute a  separate  constellation,  called  Crater,  or  the  Cup,  which 
is  on  Hydra;  but  it  is  unimportant.  The  triangle  N.  E.  of  Cor 
Hydra  should  be  found.  The  equinoctial  crosses  this  constel- 
lation north  of  this  triangle  and  Cor  Hydra. 

T  t;  r»  ivr  A  Tr»T?  *  ^e  Greater  Lion.      Leo  can  be 

LEO  MAJOR.     Z.  C.    found  in  sprjng  without  knowing 

any  other  constellation  than  the  Great  Dipper,  from  which  it  is 
S.  W.  The  fine  ist  m.  star  Regulus  will  be  easily  recognized. 
It  is  not  very  near  the  Dipper,  but  no  other  ist  m.  star  comes 
between  the  two.  Regulus  is  in  the  handle-end  of  the  Sickle, 
which  the  student  is  supposed  to  have  drawn  when  he  looks  for 
Regulus.  There  is  another  figure,  a  right  triangle,  belonging  to 
Leo,  lying  W.  of  the  Sickle,  and  containing  a  2d  m.  star,  Denebola. 
Regulus  is  nearly  on  the  ecliptic,  the  only  ist  m.  star  which 
marks  it.  Leo  can,  of  course,  be  easily  learned  after  Cancer, 
Gemini,  or  Virgo,  if  they  are  already  found.  It  is  between 
Cancer  and  Virgo,  between  Gemini  and  Virgo.  (See  page  81.) 
The  Lesser  Lion.  This  is  a  very  incon- 
spicuous constellation.  One  3d  m.  star 

*  This  constellation  is  usually  called  simply  Leo. 


LEO  MINOR. 


ASTRONOMY  BY  OBSERVATION. 


LEPUS. 


can  be  found  by  the  student  between  Leo  Major  and  the  Great 
Dipper.     This  is  the  only  bright  star  in  Leo  Minor. 

The  Hare.  This  can  be  found  after 
Orion,  from  which  it  lies  directly  S. 
There  are  two  figures  on  the  map.  Nearest  Orion  there  is  a 
row  of  three  stars ;  and  S.  of  that,  an  irregular  figure  like  a 
woman's  hanging  sleeve. 

T  TDD         -7    r*         or  tne   Balances.     This   can   be   found 
LIoKA,    Z.  C., 


LYRA. 


i0;  as   it   ]ies   be_ 

tween  them.  There  is  a  somewhat  irregular  quadrilateral  with 
two  2d  m.  stars  ;  the  others  are  4th  m.  The  ecliptic  passes 
nearly  through  the  most  southern  of  the  2d  m.  stars,  and  be- 
tween the  two  4th  m.  One  of  the  two  2d  m.  stars  marking  the 
ecliptic  is  in  Libra  ;  the  other  is  in  Scorpio. 

The  Harp.     In  late  spring  a  fine  ist  m. 

star  of  a  bluish  appearance  is  seen  in  the 
N.  E.  This  is  Vega  in  Lyra.  Lyra  can  be  found  after  Her- 
cules or  Cygnus.  It  is  E.  of  Hercules,  W.  of  Cygnus,  and  lies 
not  far  from  the  western  border  of  the  Milky- Way.  Two  4th 
m.  stars  very  near  it  form  a  very  small,  nearly  isosceles,  triangle. 
One  of  these,  which  young  eyes  can  see  is  double,  is  f  Lyrae, 
the  double  star  mentioned  in  the  last  chapter  of  this  book. 
There  are  two  3d  m.  stars  a  little  farther  off.  The  constella- 
tion is  very  small. 

The  Fly.     This  is  a  very  small   group 

containing  one  3d  m.  star.  It  lies  S.  of 
Algol,  in  Perseus ;  S.  E.  from  Triangula ;  N.  E.  from  Aries. 


MUSCA. 


NAVIS. 


OPHIUCHUS, 


See  ARGO. 

or   Serpentarius.      The    Serpent-Bearer. 


This  is  usually  divided  into  two  constel- 
lations, viz.,  Ophiuchus  and  Serpens.  They  are  here  treated  as 
one.  Ophiuchus  is  best  learned  after  Hercules,  or  Bootes  and 
Corona,  Aquila  or  Scorpio.  It  is  a  very  large  constellation, 
and  contains  four  zd  m.  stars.  It  is  just  W.  of  the  Milky- Way, 
which  aids  to  find  it.  The  most  western  stars  run  between  the 
divided  part  of  the  Milky-Way.  Hercules  and  Ophiuchus  run 
so  into  each  other  that  it  is  difficult  to  separate  them  into  two 
figures  ;  and,  after  drawing  each  separately,  they  should  be 
drawn  on  the  same  paper. 

DRTOM  This  is  the  most  brilliant  constellation 

in  the  heavens.     It  can  be  found  after 

knowing  Gemini,  Sirius,  or  Taurus  ;  or  it  can  easily  be  found, 
without  any  other  guide  than  its  own  conspicuous  figure,  as  soon 
as  the  student  can  draw  it  from  memory.  In  the  early  winter 
it  is  seen  in  the  E.  at  dark;  it  passes  the  meridian  at  dark 
about  the  close  of  February  and  the  beginning  of  March  ;  and 
after  that  it  is  seen  in  the  W.  until  late  in  spring.  When  it 
passes  the  meridian,  it  must  be  looked  for  a  good  deal  S.  of  the 
observer.  There  are  two  ist  m.  stars,  Rigel  and  Betelguese. 
Betelguese  is  farther  N.  than  Rigel,  and  reddish.  Another  of 
the  four  stars  forming  the  quadrilateral  is  zd  m.  This  is  called 
Bellatrix.  Within  the  quadrilateral  are  found  three  zd  m.  stars 
in  a  line  running  S.  E.  and  N.  W.  They  are  said  to  be  in  the 
belt  of  Orion.  There  runs  a  line  of  stars  S.  from  these  which 


PEGASUS. 


are  said  to  be  in  the  sword-handle.  One  of  these  looks  hazy. 
It  is  the  nebulous  star  mentioned  in  Chapter  XI.  The  small 
triangle  which  lies  in  the  N.  of  the  quadrilateral  is  said  to  be 
in  the  neck  of  Orion.  (See  page  81.) 

Pegasus  can  be  learned  after  Aquarius,  Cyg- 
nus, or  Andromeda  ;  or  it  can  easily  be  recog- 
nized by  its  own  conspicuous  figure,  if  the  observer  has  drawn 
it.  Four  zd  m.  stars  form  a  large  square,  not  exactly  regular, 
called  the  Great  Square  of  Pegasus  ;  and  to  the  N.  W.  angle  is 
attached  a  triangle  of  smaller  stars,  which  aids  in  identifying 
it.  The  N.  E.  star  of  the  Great  Square  really  belongs  to  An- 
dromeda, but  helps  to  complete  the  Great  Square.  In  Septem- 
ber Pegasus  is  seen  in  the  E.  at  dark  ;  it  passes  the  meridian  at 
dark  in  early  winter,  when  it  is  nearly  overhead  ;  and  after 
that  it  is  seen  in  the  W.  at  dark  until  March.  S.  W.  from  the 
Square  there  is  another  figure  which  the  student  must  learn  by 
drawing  it,  after  he  knows  the  Square.  (See  page  81.) 

and  Medusa's  Head.  This  is  very  easily 
learned  after  Cassiopeia,  Auriga,  or  Taurus. 
It  lies  in  the  Milky-  Way.  Part  of  Perseus  is  a  portion  of  a  cir- 
cle called  the  Segment  of  Perseus.  Its  concave  side  turns  N.  E. 
Besides  this  a  curved  branch  runs  S.  W.,  and,  at  the  end  of  the 
branch,  two  stars  make  a  sudden  bend.  E.  of  this  curve  there 
are  two  stars,  one  a  not  very  bright  2d  m.  The  two  are  near 
each  other,  and  the  2d  m.  star  is  the  remarkable  variable  star 
Algol,  or  Beta  Persei.  An  account  of  it  is  given  in  Chapter  XL 
There  is  a  zd  m.  star  in  the  Segment  of  Perseus  called  Algenib. 
It  is  at  the  junction  of  the  segment  and  branch. 

The    Fishes.      This   constellation    is   best 

7 


PERSEUS 


learned  after  Pegasus,  Aquarius,  and  An- 
dromeda. Only  one  star  is  as  bright  as  3d  m.  ;  but,  with  a 
little  care,  the  constellation  is  easily  found,  as  it  is  not  indis- 
tinct. The  student  must  find  first  a  small  hexagon,  not  quite 
regular,  of  4th  and  5th  m.  stars,  not  indistinct.  From  the  hexa- 
gon a  chain  of  faint  stars  runs  nearly  E.  and  W.,  terminating  in 
a  3d  m.  star.  From  this  another  chain  runs  nearly  N.,  and 
just  S.  of  Andromeda  it  meets  a  cluster  of  faint  stars.  This 
last  cluster  is  the  Northern  Fish,  or  Piscis  Borealis.  The  hexa- 
gon is  the  Western  Fish,  or  Piscis  Occidentalis.  The  two  most 
southern  stars  of  the  hexagon  are  nearly  on  the  equinoctial,  and 
a  very  little  E.  of  them  is  the  point  where  the  sun  is  found  at 
the  Vernal  Equinox  on  March  2  ist.  If  a  line  were  drawn  S. 
through  the  two  eastern  stars  of  the  Square,  it  would  be  a  little 
E.  of  the  Vernal  Equinox.  On  the  maps  of  Pisces  made  by 
students,  the  point  should  be  marked  V.  E.  There  are  three 
4th  m.  stars  in  the  chain  running  E.  from  the  Western  Fish. 
The  ecliptic  runs  just  S.  of  them,  and  crosses  the  chain  or  rib- 
bon, as  it  is  sometimes  called,  just  W.  of  them.  The  chain 
connected  with  the  Northern  Fish  has  in  it  one  4th  m.  star, 
and  farther  S.  two  5th  m.  stars.  Between  the  two  5th  m.  stars 
the  ecliptic  crosses  that  chain.  The  3d  .m.  star  joining  the 
chains  is-a  very  little  N.  of  the  equinoctial. 

The     Southern     Fish.      This 
PISCIS  AUSTRALIS. 


already   been     described 


in  Aquarius. 


APPENDIX  A.— DESCRIPTION  OF  CONSTELLATIONS. 


SAGITTARIUS.   Z.C. 


The  Archer.     This  constellation 


SCORPIO.     Z.  C. 


is  seen  in  summer  and  autumn, 
and,  when  Scorpio  is  known,  it  can  be  found  from  it  ;  but  it  is 
also  very  easily  distinguished,  because  it  is  at  the  southern  and 
brightest  part  of  the  Milky-Way.  First  a  small  figure  must  bo 
found,  like  a  dipper  turned  upside  down.  This  is  called  the 
"  Milk-Dipper."  S.  W.  of  this,  and  just  in  the  Milky-Way,  is 
a  small  right  triangle  containing  a  2d  m.  star.  N.  E.  from  the 
Dipper  there  is  an  irregular  cluster  of  very  small  stars,  not 
bright,  but  important,  because  the  ecliptic  passes  through  them. 
Nearly  N.  from  the  right  triangle  is  the  point  where  the  sun  is 
found  on  December  22d,  at  the  winter  solstice.  The  point 
should  be  marked  on  the  maps  of  students. 

The  Scorpion.   This  constellation 

can  be  found  in  the  S.  on  any 
clear  summer  evening,  first  by  the  brilliant  ist  m.  star  Antares  ; 
and  next  by  its  peculiar  shape,  outlined  by  many  3d  m.  stars.  An- 
tares is  very  red,  and  nearer  the  southern  horizon  than  any  ist  m. 
star  seen  in  summer,  so  there  is  no  possible  chance  to  mistake  it. 
There  is  also  a  2d  m.  star,  and  the  ecliptic  passes  nearly  through 
it.  This  is  one  of  two  zd  m.  stars  which  mark  the  course  of 
the  ecliptic.  The  other  is  in  Libra,  just  W.  of  Scorpio.  If  the 
student  knows  Libra,  Sagittarius,  or  Ophiuchus,  Scorpio  can  be 
learned  from  them.  Scorpio  is  E.  of  Libra,  W.  of  Sagittarius, 
S.  of  Ophiuchus.  After  drawing,  it  is  impossible  not  to  recog- 
nize it,  though  it  is  so  near  the  horizon  that  the  tail  is  often 
a  little  obscured  through  smoke.  N.  of  the  tail  there  are 
four  small  stars  in  line,  and  the  most  northern  is  nearly  on 
the  ecliptic.  These  stars  really  belong  to  the  constellation 
Ophiuchus,  but  are  mentioned  here  because  they  aid  us  in 
tracing  the  ecliptic.  (See  page  81.) 

The  Bull.   Taurus  can  be  learned 

c,          s-\   •  »        •  *-~*         *    • 

after  Orion,  Auriga,  Gemini,  or 
Aries.  It  is  N.  of  Orion,  W.  or  S.  W.  of  Gemini,  S.  W.  of  Auriga, 
E.  or  N.  E.  of  Aries.  There  is  a  cluster  of  small  stars  popularly 
known  as  the  "  Seven  Stars."  They  are  all  small,  but  united  they 
become  conspicuous.  These  are  the  Pleiades.  S.  E.  from  the 
Pleiades  there  is  a  figure  like  the  letter  V  containing  a  reddish 
ist  m.  star,  Aldebaran.  The  V  is  called  the  Hyades.  If  a  line 
be  supposed  to  join  the  Pleiades  and  the  Hyades,  a  small  group 
of  four  sth  m.  stars  in  a  line  will  be  just  E.  of  it.  The  most 
southern  one  of  these  is  nearly  on  the  ecliptic,  which  runs  be- 
tween the  Pleiades  and  Hyades. 


7 

/  ,  . 


TRIANGULA. 


The  Triangle.  This  can  best  be  learned 
after  Andromeda  and  Perseus.  It  is  a 
slender,  nearly  isosceles,  triangle,  lying  between  Perseus,  An- 
dromeda, Pisces,  and  Aries.  It  must  not  be  mistaken  for  the 
triangle  in  Aries,  which  contains  a  2d  m.  star,  and  is  nearly  S. 
from  it.  The  triangle  in  Aries  is  not  isosceles. 

MA  TOR      ThC  Great  Bear'     Th£  map  giveS  tW° 
IV1AJUK. 


Dipper,  and  a  row  of  stars  in  twos  S.  W.  from  the  Dipper.  The 
Dipper  is  sufficiently  described  in  Chapter  I.  The  names  of 
the  stars  of  the  Dipper,  given  in  order,  beginning  with  the  han- 
dle-end, are  Benetnasch,  Alcor,  Alioth,  Megres  (joining  bowl 
and  handle),  Phad,  Merach  (the  last  two  in  the  bottom  of  the 
bowl),  and  Dubbhe.  It  is  not  desirable  to  try  to  make  students 
learn  these,  unless  they  study  for  a  long  time.  There  will 
inevitably  be  confusion  among  so  many  names.  The  row  of 
stars  in  the  feet  is  not  well  seen  except  in  spring  and  sum- 
mer. It  is  not  desirable  to  teach  it  when  the  Dipper  is  first 
learned.  After  the  more  important  constellations  are  learned 
there  can  be  a  review,  if  there  is  time,  and  this  figure  can  be 
learned.  These  stars  are  in  the  feet  of  the  Great  Bear. 


• 


URSA  MINOR. 


The   Lesser  Bear.     This  contains  but 


\Tforr\ 
VIRGO.     Z.  C. 


one  important  figure,  the  Little  Dipper, 
and  that  is  fully  described  in  Chapter  I.  Polaris  is  2d  m.  ;  the 
Guardians  are  3d  m. 

^e  Virgin.     This  can  best  be  learned 
after  LeQ^  Libraj  or  B00-tes-    it  is  nearly 

E.  (a  little  S.  E.)  from  Leo,  nearly  W.  (a  little  N.  W.)  from 
Libra,  and  it  is  S.  W.  from  Bootes.  The  ist  m  star  Spica 
helps  to  identify  it.  After  drawing,  the  figure  is  very  easily 
identified.  There  is  a  figure  resembling  somewhat  a  chair 
turned  back,  only  the  seat  of  the  chair  projects  too  much.  The 
stars  of  the  figure  are  3d  m.,  except  Spica  and  a  faint  4th  m., 
through  which  the  ecliptic  passes  and  gives  it  importance. 
There  is  another  faint  5th  m.  star  in  Virgo  (but  not  in  this  figure) 
through  which  the  ecliptic  passes.  It  is,  on  the  map,  between 
the  figure  and  Libra.  This  star  is  said  to  be  in  the  feet  of  the 
Virgin.  The  ecliptic  passes  just  S.  of  three  of  the  3d  m.  stars, 
which  therefore  mark  its  course.  The  point  where  the  ecliptic 
crosses  the  equinoctial  is  the  point  where  the  sun  is  found  Sep- 
tember 2  ist.  This  is  therefore  the  point  of  the  Autumnal  'Equi- 
nox, and  it  should  be  marked  in  all  copies  of  this  map  of 
Virgo,  A.  E 


86 


ASTRONOMY  BY  OBSERVATION. 


APPENDIX    B. 

THE    TELESCOPE. 

(Taken  chiefly  from  Lockyers  "  Astronomy.") 


Construction. — The  telescope  is  a  combination  of  lenses. 
The  principle  involved  in  its  construction  is  simply  an  extension 


FIG.  108 


Construction  of  the  Astronomical  Telescope. 

of  that  exhibited  in  the  structure  of  the  eye.  In  the  eye  nearly 
parallel  rays  fall  on  a  lens,  and  this  lens  throws  an  image.  In 
the  telescope  nearly  parallel  rays  fall  on  a  biconvex  lens  ;  this 
lens  throws  an  image,  and  then  another  lens  enables  the  eye  to 
form  an  image  of  the  image  by  rendering  the  rays  again  par- 
allel. These  parallel  rays  enter  the  eye  just  as  they  do  in  ordi- 
nary vision.  In  Fig.  108,  for  instance,  let  A  represent  the  front 
lens,  called  the  object-glass,  because  it  is  nearest  to  the  object 
viewed ;  let  C  represent  the  other,  called  the  eye-piece,  because 
it  is  nearest  the  eye;  and  let  B  represent  the  image  of  a  distant 
arrow,  the  rays  from  which  are  seen  falling  on  the  object-glass 
from  the  left.  These  rays  are  refracted,  and  we  get  an  inverted 
image  at  the  focus  of  the  object-glass,  which  is  also  the  focus  of 
the  eye-piece.  The  rays  leave  the  eye-piece  adapted  for  vis- 
ion as  they  are  when  they  fall  on  the  object-glass  ;  the  eye 
can  therefore  use  them  as  well  as  if  no  telescope  had  been 
there. 

The  efficiency  of  the  telescope  depends  on  two  things — its 
illuminating  power  and  its  magnifying  power.  First,  as  to  its 
illuminating  power.  The  object-glass,  being  larger  than  the 
pupil  of  our  eye,  receives  more  rays  than  the  pupil.  If  its  sur- 
face be  a  thousand  times  greater  than  that  of  the  pupil,  for 
instance,  it  receives  a  thousand  times  more  light ;  and,  conse- 
quently, the  image  of  a  star  formed  at  its  focus  is  nearly  a  thou- 
sand times  brighter  than  that  thrown  by  the  lens  of  our  eye  on 
the  retina. 

The  magnifying  power  depends  on  two  things :  first,  it  de- 
pends on  the  focal  length  of  the  object-glass ;  next,  the  magni- 
fying power  of  the  eye-piece  is  to  be  taken  into  account.  This 
varies  according  to  the  eye-piece  used,  the  ratio  of  the  focal 
length  of  the  object-glass  to  the  eye-piece  giving  its  exact 
amount.  Thus,  if  the  focal  length  of  the  object-glass  is  one 
hundred  inches  and  that  of  the  eye-piece  one  inch,  the  tele- 
scope will  magnify  one  hundred  times.  But,  unless  the  illumi- 


nating power  is  good  and  a  perfect  image  is  formed,  a  high 

magnifying  power  is  useless.     If  the  object-glass  does  not  per- 
form its  part  properly,  the  image  will  be  blurred. 

The  telescope-tube  keeps  the  object-glass  and 
the  eye-piece  in  their  proper  positions  ;  and  the 
eye-piece  is  furnished  with  a  draw-tube,  which 
allows  its  distance  from  the  object-glass  to  be 
varied. 

Mountings. — An  astronomer  uses  the  tele- 
scope for  different  kinds  of  work.  Accordingly, 
he  mounts,  or  arranges,  his  instrument  in  several 
different  ways.  When  he  desires  to  watch  the 

heavenly  bodies,  the  only  essential  is  that  the  instrument  should 

be  so  arranged  as  to  command  every  portion  of  the  sky.     The 

best  mounting  for  this  purpose  is  shown  in  Fig.    109.      With 

such    an  instrument, 

called  an  equatorial,  FlG'  109' 

a  heavenly  body  may 

be  followed  from  its 

rising  to  its  setting, 

the     proper    motion 

being  communicated 

by     machinery.      In 

this         arrangement 

there  is  a  stror.g  iron 

pillar    supporting    a 

head-piece,  in  which 

is  fixed  the  polar  axis 

of  the  instrument  par- 
allel  to    the    axis  of 

the  earth.     This  po- 
lar axis   is   made  to 

turn    round   once   in 

twenty  -  four    hours. 

The  machinery  turns 

the  telescope  in   one 

direction  just  as  fast 

as  the  earth  moves  in 

the   other,   and  thus 

the     instrument      is 

kept    all    the    times 

fixed  on  a  heavenly 

body.      It  is   incon- 
venient   to     fix    the 

telescope  on   the  polar  axis,  as  its  range  is  then  limited  ;  it  is 

fixed,  therefore,  to  an  axis  at  right  angles  to  the  polar  axis. 


NORTHERN    HEMISPHERE 


CELESTA 

Showing  the   Position  c 
the   most   Irr 


Xiv, 


XIII 


NOTE. — The  astronorr 
to  the  names  of  stars  de 
constellation  in  the  order 
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an  observer  detects  occasi 
stars  also  have  varied  in 
Pollux  in  Gemini,  which  i 
while  Castor  is  a.  The 
can  not  easily  be  change 


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APPENDIX  C.— OBSERVATION  OF  METEORS  AND   COMETS. 


APPENDIX    C 

OBSERVATION     OF     METEORS     AND     COMETS. 


MR.  E.  E.  BARNARD,  of  Vanderbilt  Observatory,  Nash- 
ville, Tennessee,  a  well-known  observer  of  comets,  gave  to  the 
author's  pupils  some  simple  directions  for  observation  of  mete- 
ors and  comets,  without  instruments.  They  are  of  wider  use,  so 
the  substance  of  them  is  here  given. 

Meteors. — On  any  evening  when  you  see  them,  note  care- 
fully at  what  point  among  the  stars  each  appears,  at  what  point 
it  disappears.  Trace  these  paths,  as  you  learn  each,  on  a  chart 
or  globe  of  the  heavens.  If  you  are  fortunate  enough  to  see  a 
number,  you  will  find  that  some  of  the  paths  intersect.  The 
point  of  intersection  is  the  radiant,  and  you  will  have  seen  a 
shower.  If  you  become  an  accurate  observer,  some  of  the  peri- 
odicals devoted  to  the  stars  may  be  glad  to  publish  your  report. 
The  record  should  be  about  as  follows  :  ist  m.  meteor,  appeared 
2%°  N.,  3°  W.  of  Vega  ;  disappeared  4°  due  S.  of  Altair;  time 
of  flight,  iY8  second  ;  color  reddish,  faint  train,  permanent  for 
5  seconds,  exploded  with  several  red  sparks.  To  such  a  report 
is  added  the  date,  the  mean  time,  and  the  exact  location  of  the 
observer.  Any  change  of  color  in  the  meteor  during  flight,  or 
at  the  time  of  bursting,  should  be  noticed  and  recorded.  If 
the  student  chances  to  see  a  large  meteor,  there  is  a  special 
value  in  his  report,  for  the  same  object  may  be  seen  in  some 
other  place,  and  the  two  observations  will  enable  astronomers 
interested  in  the  study  of  meteors  to  tell  all  about  it. 

To  this,  the  author  adds  a  few  words  in  regard  to  the  obser- 
vation of  the  Leonids,  or  November  meteors.  If  the  observer 
sees  at  once  the  trains  of  a  number  of  meteors,  the  radiant 
point  or  intersection  is  evident  without  tracing  on  a  map.  On 
November  I4th,  the  constellation  Leo  is  on  the  eastern  horizon 
at  midnight,  and  the  sun,  of  course,  is  on  the  meridian  below 
the  horizon.  A  line  drawn  from  the  observer  to  the  sun,  and 
another  to  the  point  where  the  ecliptic  and  eastern  horizon 
intersect  in  Leo,  would,  it  is  evident,  make  a  right  angle.  At 
midnight,  the  ellipse  which  is  the  earth's  orbit  is  below  us, 


except  the  point  we  are  on  (for  the  orbit  must  always  lie  on  the 
same  side  of  us  as  the  sun).  Thus  it  is  plain  that  the  line  to 
the  eastern  horizon  is  a  tangent  to  the  earth's  orbit. 

Now,  the  student  must  remember  that  the  earth's  motion, 
like  that  of  a  key  revolved  by  the  hand  on  a  string,  would  at 
any  moment  carry  it  in  the  direction  of  a  tangent  to  its  orbit 
but  for  the  attraction  at  the  sun  holding  it  fast  and  continually 
bending  the  straight  line  into  a  curve  or  ellipse.  Therefore,  at 
midnight,  November  i4th,  the  earth  is  moving  directly  toward 
Leo,  and  the  meteors  are  coming  from  the  quarter  toward  which 
the  earth  is  moving  at  that  time.  As  we  only  see  them  on  or 
near  November  i4th,  it  is  evident  that  the  paths  of  the  earth 
and  the  meteors  are  not  identical,  but  intersecting  paths.  As 
they  come  to  meet  us,  their  motion  is  retrograde,  or  from 
east  to  west.  When  the  earth's  rotation  on  her  axis  makes 
Leo  appear  to  move  west,  the  radiant  point  seems  to  move 
with  it. 

Comets.— In  order  to  observe  a  comet  with  intelligence, 
the  corrxet's  position  among  the  stars  must  be  noted,  and  also 
the  position  of  the  sun  at  the  time.  Then  it  is  easy  for  the  ob- 
server to  note  its  motion  toward  and  from  the  sun,  and  the 
changes  it  undergoes  in  approaching  and  receding  from  him, 
especially  the  changes  in  its  tail.  Its  path  in  regard  to  the 
ecliptic  should  be  noted,  as  we  thus  gain  some  idea  of  the  plane 
in  which  it  moves.  Mr.  Barnard  says,  "In  the  case  of  a  large 
comet,  naked-eye  observations  may  be  worth  recording.  The 
time  of  the  observation  should  be  given  to  the  nearest  minute. 
The  limits  and  general  position  of  the  tail  should  be  carefully 
sketched,  and  that  of  the  head,  with  notes  as  to  curvature  of 
tail,  brightest  parts,  etc.  Any  markings  on  the  tail,  such  as 
dark  streaks,  etc.,  should  be  carefully  traced.  Such  work,  done 
accurately,  will  be  valuable  work.  Above  all  things,  students 
should  be  taught  to  be  very  careful,  and  have  no  uncertainties 
without  fully  stating  them" 


INDEX. 

(The  numbers  refer  to  articles,  not  to  pages.) 


NOTE. — In  making  this  index,  the  author  designed  that  it  should  both  answer  the  object  of  an  index  and  serve  as  a  list  of  topics  for  review.  The  order 
in  which  it  is  wise  to  treat  a  subject  for  beginners,  necessarily  separates  subjects  with  some  connections.  It  is  nearly  always  best  to  review  in  a  different 
order  where  it  can  conveniently  be  managed.  It  brings  out  relations  which  have  been  neglected,  and  excites  the  minds  of  students,  to  whom  review  is  tire- 
some. The  author,  in  teaching  other  books,  has  found  a  good  index  useful  for  review,  and  has  employed  this  experience  in  making  this  index.  Teachers 
who  have  never  tried  the  plan  of  adopting  a  different  order  for  review  are  recommended  to  make  trial.  Where  the  plan  seems  to  make  too  much  repetition, 
there  may  be  omissions. 


Absorption  of  light,  by  vapors,  174  ;  by  the  at- 
mosphere of  Mars,  198  ;  by  atmospheres  of  the 
fixed  stars,  223. 

Aerolites,  defined,  205  ;  composition  of,  206  ;  me- 
teoric iron  and  meteoric  stones,  206  ;  origin  of, 
207. 

Aldebaran,  1st  m.  star,  5  ;  color  and  composition 
of,  223. 

Algol,  or  ft  Persei,  variable  star,  228. 

Alpha,  or  a  Centauri,  annual  parallax  and  distance 
of,  224. 

Alphabet,  Greek,  6. 

Altair,  1st  m.  star,  5  ;  color  and  composition  of, 
223. 

Altitude.  The  angular  distance  of  a  heavenly 
body  above  the  horizon,  19. 

Andromeda,  nebula  in,  232 ;  new  star  in,  note, 
page  79. 

Antares,  1st  m.  star,  5  ;  color  and  composition  of, 

223-         fcsL  - 

Aphelion.  The  point  which  is  farthest  from  the 
sun  on  the  orbit  of  a  heavenly  body  revolving 
round  the  sun,  46. 

Apogee.  The  position  of  the  sun  or  moon  when 
at  its  greatest  distance  from  the  earth,  46. 

Apsides,  line  of.  A  line  joining  the  perihelion 
and  aphelion  points  of  the  earth's  orbit.  It  is 
the  major  axis  of  an  ellipse,  74. 

Aquarius,  star-cluster  in,  230. 

Ara,  a  southern  circumpolar  constellation,  235. 

Arctic  and  Antarctic  circles,  62. 

Arcturus,  1st  m.  star,  5  ;  motion  of,  222  ;  color  and 
composition,  223. 

Aries,  1st  point  of,  79. 

Asteroids,  minor  planets,  188,  192,  193. 

Axis,  celestial,  40. 

Azimuth.  This  is  angular  distance,  measured  hori- 
zontally, from  the  meridian.  It  is  measured  by 
the  arc  of  the  horizon  intercepted  between  the 
meridian  and  a  vertical  circle  passing  through 
the  body  whose  azimuth  is  sought,  19. 

Beta,    or   0   Persei,    variable    star   called    Algol, 

228'  0  J 

Betelguese,  1st  m.  star,  5  ;  motion  of,  222.  /\j>vl,~ 


Calendar.     See  "  Time,"  81-91. 

Cancer,  Pnesepe  or  Beehive  Nebula  in,  232. 

Canopus,  1st  m.  star  in  Centaurus,  5,  235. 

Capella,  1st  m.  star,  5  ;  color  and  composition  of, 
223. 

Cassiopeia,  described,  7 ;  new  star  in,  229. 

Centaurus,  cluster  of  stars  in,  230. 

Ceres,  an  asteroid,  188. 

Cetus,  variable  star  Mira  in,  228. 

Circles.  Great  circles  bisect  each  other,  52  ;  diur- 
nal circles,  18,  22  ;  small  circles,  52  ;  circles  of 
perpetual  apparition  and  disparition,  21  ;  equi- 
noctial system  of  circles,  18,  49  ;  horizon  system, 
19  ;  ecliptic  system,  48. 

Clusters  of  stars,  230. 

Colures.  Great  circles  passing  through  the  celes- 
tial poles  and  the  equinoctial  or  solstitial  points 
of  the  ecliptic,  49. 

Comets,  description  of,  209 ;  the  parts  of  comets, 
comets  of  Donati  and  Coggia,  vaporous  enve- 
lopes of  comets,  210,  211  ;  orbits  and  origin  of 
comets,  periodical  comets,  212  ;  return  of  com- 
ets, Halley's  comet,  effect  of  large  planets  on 
comets,  2:3 ;  telescopic  comets,  Biela's  comet, 
comet  of  1843,  214  ;  spectra  of  comets,  comet 
of  1881,  215  ;  numbers  of  the  comets,  Newton 
and  the  comet  of  1680,  comet  of  1811,  216. 

Conjunction.  A  planet  or  the  moon  is  in  conjunc- 
tion with  the  sun,  when  the  sun  and  earth  are  in 
line  with  it,  and  the  earth  is  not  in  the  middle, 
C4,  127  ;  superior  and  inferior  conjunctions,  148. 

Constellation,  a,  defined,  2  ;  for  account  of  all  the 
important  constellations  of  the  northern  hemi- 
sphere, see  Appendix  A,  page  80. 

Corona,  the  sun's,  185. 

Corona  Borealis,  new  star  in,  229. 

Day  and  night,  unequal,  54-61  ;  uses  of  word  day, 
83,  note  ;  sidereal  and  solar  days,  26,  83  ;  calen- 
dar days,  84. 

Declination.  This  is  angular  distance  north  or 
south  from  the  equinoctial,  18. 

Deimos,  moon  of  Mars,  198. 

Density,  of  sun,  169  ;  of  planets,  191. 

Disks,  of  fixed  stars,  225. 


Distances,  of  sun  from  earth,  193  ;  of  planets  from 
the  sun,  193  ;  comparative  distances  from  the 
earth  at  opposition  or  conjunction  of  Mars,  Ju- 
piter, and  Saturn,  127,  138  ;  variation  in  a  plan- 
et's distances  from  the  earth,  127  ;  distances  of 
fixed  stars  from  the  earth,  41,  4",,  224. 

Dorado  (sword-fish),  southern  circumpolar  con- 
stellation, 235. 

Earth,  the,  her  diurnal  rotation,  9,  10  ;  plane  and 
direction  of  diurnal  motion,  II,  12  ;  annual  mo- 
tion, 35,  36  ;  inclination  of  axis,  39,  40  ;  the 
earth  and  the  ecliptic,  34,  47  ;  the  earth's  orbit, 
43,  46 ;  the  earth  in  aphelion  and  perihelion, 
46  ;  inequality  of  her  days  and  nights,  54-62  ; 
her  seasons,  62-72  ;  inequalities  of  her  annual 
motion,  72,  73  ;  revolution  of  line  of  apsides, 
75  ;  revolution  of  equinoctial  points  and  of  the 
poles,  76,  77  ;  her  form,  189  ;  diameter  and  vol- 
ume, 190  ;  density,  191 ;  distance  from  the  sun, 

193- 

East  and  west,  12. 

Eclipses,  obseivation  and,  104;  account  of,  112- 
118  ;  eclipses  of  moon,  114;  of  sun,  115  ;  of 
Jupiter's  moons,  199. 

Ecliptic,  the.  A  great  circle  of  the  celestial  sphere 
the  plane  of  which  passes  through  the  centers  of 
the  earth  and  sun  and  intersects  the  plane  of  the 
equinoctial  at  an  angle  of  23^°.  Ecliptic  de- 
scribed, 32  ;  sun's  motion  on  it,  33,  47  ;  relation 
to  the  equinoctial,  32  ;  the  ecliptic  as  seer  at 
dark  and  the  earth's  annual  motion,  42  ;  aspects 
of  the  ecliptic,  50 ;  proved  a  great  circle,  52  ; 
how  it  is  traced  on  the  heavens,  53  ;  the  eclip- 
tic and  the  moon's  path,  99-102  ;  the  ecliptic 
and  the  moon's  crescent,  105  ;  harvest  moon  and 
the  ecliptic,  108,  109  ;  eclipses  and  ecliptic,  117  ; 
superior  planets  and  ecliptic,  128,  139,  193  ;  Ve- 
nus and,  144,  150,  193  ;  Mercury  and,  155. 

Ecliptic  system  of  circles,  48,  49. 

Elements,  chemical,  in  the  sun,  177  ;  in  fixed 
stars,  223  ;  in  meteors,  205,  206. 

Ellipses,  46,  192,  208,  212. 

Epsilon  (or  e )  Lyras,  double  star,  227. 

Equinoctial,  the.     A  great  circle  of  the  celestial 


INDEX. 


sphere  perpendicular  to  its  axis,  18  ;  angle  with 
ecliptic,  32. 

Equinoctial  system  of  circles,  18. 

Equinoxes,  vernal  and  autumnal,  49. 

Eta  ^r  7j)  Argus,  variable  star,  228. 

Experiments  illustrating  the  diurnal  motion  of  the 
stars,  8  ;  to  investigate  the  earth's  annual  revo- 
lution, 38,  39,  44  ;  to  illustrate  the  inclination 
of  the  celestial  axis,  40  ;  to  show  the  plane  of 
the  earth's  motions,  40 ;  to  show  the  effect  of 
oblique  heat-rays,  67  ;  to  illustrate  the  preces- 
sion of  the  equinoxes,  77  ;  to  illustrate  the  retro- 
grade motions  of  superior  planets,  135  ;  to  illus- 
trate the  motions  of  inferior  planets,  147. 

Fixed  stars.     See  Stars. 

Fomalhaut,  1st  m.  star,  5. 

Fraunhofer's  lines.     Name  of   the  dark  lines  of 

the  solar  spectrum,  taken  from  their  discoverer, 

174,  223. 

Galaxy,  the,  the  Milky-Way,  6,  231. 

Globes;  celestial,  their  uses,  17. 

Gravitation,  attraction  of,  160-163. 

Great  Dipper,  description  of,  7  ;  motion  of  the 
stars  in,  221.  See  constellation  Ursa  Major  in 
Appendix  A. 

Gregorian  Calendar,  87. 

Grus,  the  Crane,  southern  circumpolar  constella- 
tion, 235. 

Guardians  of  the  Pole,  7. 

Harvest  moon,  109. 

Heat,  obliquity  of  sun's  rays  and  heat,  63,  68  : 
the  degree  of  the  sun's  heat,  171. 

Hemispheres.  The  meridian  of  every  place  di- 
vides the  celestial  sphere  into  eastern  and 
western  hemispheres.  Their  area  constantly 
changes,  56.  The  equinoctial  divides  the  celes- 
tial sphere  into  northern  and  southern  hemi- 
spheres. Their  area  does  not  change.  See  maps 
v  and  vi. 

Horizon,  celestial.  This  is  a  great  circle  of  the 
celestial  sphere  of  which  every  point  is  90° 
from  the  zenith,  19,  45  ;  the  horizon  and  the 
poles,  20. 

Horizon  system  of  circles,  19. 

Hour  circles.  These  are  great  circles  passing 
through  the  celestial  poles  and  perpendicular  to 
the  equinoctial,  18. 

Hydrus,  southern  circumpolar  constellation,  235. 

Inclination  of  the  earth's  axis,  39,  40. 
Inferior  conjunction,  148. 
Inferior  planets,  140,  188. 
Illumination  of  earth,  62. 

Juno,  a  minor  planet,  188. 

Jupiter,  superior  and  major  planet — how  to  find 
him,  120-124  ;  motions  as  superior  planet,  124- 
140 ;  his  retrograde  motion,  136  ;  comparative 
distance  from  the  earth  at  opposition,  127,  138  ; 
his  form,  189  ;  diameter  and  volume,  190  ;  den- 
sity, 191 ;  orbit,  192,  193  ;  distance  from  sun, 
193  ;  period  of  revolutions,  194 ;  telescopic  as- 
12 


pect  and  physical  condition,  moons,  199  ;  Jupi- 
ter and  comets,  212,  213. 

Kepler's  laws,  159  ;  his  star,  229. 

Latitude,  celestial,  angular  distance  north  or  south 

from  the  ecliptic,  48,  80. 

Leonids,  meteor  train  revolving  round  the  sun,  208. 
Librations  of  the  moon,  107. 
Light,  of  sun,  170  ;  of  fixed  stars,  224. 
Little  Dipper,  7.     See  Ursa   Minor,  in  Appendix 

A,  page  80. 
Longitude,  celestial.     Angular  distance  measured 

on  the  ecliptic  east  from  the  first  point  of  Aries, 

48,  80. 

Magnitudes  of  fixed  stars  (comparative),  3,  5  ; 
(absolute),  225: 

Major  planets,  186,  187. 

Mars,  superior  and  major  planet  —  how  to  find  him, 
120-124  !  motions  as  superior  planet,  124-140  ; 
variation  in  apparent  diameter  and  distance 
from  the  earth,  127  ;  comparative  distance  from 
the  earth  at  opposition,  138  ;  retrograde  move- 
ment, 136  ;  orbit  and  parallax,  167  ;  phases, 
note  on  page  47  ;  form,  189  ;  diameter  and  vol- 
ume, 190  ;  density,  191  ;  orbit,  192,  193  ;  plane 
of  motion  and  distance  from  the  sun,  193  ;  pe- 
riod of  revolutions,  154  ;  telescopic  appearance, 
physical  condition,  moons,  198. 

Measurement,  celestial,  165 

Mercury,  inferior  major  planet  —  how  to  find  him, 
155  ;  motions  as  inferior  planet,  140-156;  phases, 
155  ;  form,  189  ;  volume  and  diameter,  190  ; 
density,  191;,  orbit,  155,  1^2;  plane  of  motion 
and  distance  from  the  sun,  193  ;  period  of  revo- 
lutions, 194  ;  telescopic  aspect,  196. 

Meridian,  the,  19. 

Meteoroids,  defined,  207  ;  account  of,  203-209  ; 
meteors,  204  ;  meteoric  stones  and  iron,  206  ; 
numbers  of,  207  ;  showers  of,  208  ;  Leonids  and 
Perseids,  208. 

Milky-Way,  6,  231. 

Minor  planets,  186,  188. 

Mira,  variable  star  in  Cetus,  228. 

Moon,  the,  opportunities  to  observe,  91  ;  apparent 
diurnal  revolution,  92  ;  real  or  proper  motion, 
93  ;  revolution  round  the  earth,  opposition  and 
conjunction,  94  ;  moon's  orbit,  and  her  varia- 
tion in  apparent  size,  96  ;  sidereal  and  synodical 
revolution,  97  ;  motion  round  sun,  98  ;  moon's 
path  among  the  stars,  99  ;  nodes,  102  ;  phases 
and  quarters,  103  ;  eclipses  and  observation,  104  ; 
position  of  crescent,  105  ;  rotation  on  axis,  106  ; 
librations,  107;  times  of  rising,  108  ;  harvest 
moon,  109  ;  north  and  south  motion,  no  ;  revo- 
lution of  nodes,  in  ;  eclipses,  112-118;  dis- 
tance from  the  earth,  193  ;  telescopic  aspect  and 
physical  condition,  202. 


Nadir.  The  nadir  .is  the  point  of  the  celestial 
sphere  under  our  feet.  It  is  one  pole  of  the 
celestial  horizon,  19. 

Nebulae  ;  definition  ;  nebulas  in  Andromeda,  Can- 
cer, and  Orion  ;  planetary,  ring-shaped,  oval  and 
spiral  nebula;  ;  spectra  of  nebulae,  232. 


Nebular  theory,  234. 

Neptune,  major  and  superior  planet  ;  its  discov- 
ery, 161  ;  motions  as  a  superior  planet  described, 
123-140  ;  form,  189  ;  diameter  and  volume,  190  ; 
density,  191  ;  orbit,  192  ;  plane  of  orbit  and  dis- 
tance from  the  suli,  193  ;  period  of  revolutions, 
194  ;  physical  condition  and  telescopic  aspect, 
moons,  201,  234. 

New  style,  88,  89 

Newton,  Sir  Isaac,  160,  162. 

Nodes.  These  are  points  where  the  orbits  of  bod- 
ies revolving  round  the  sun  cross  the  plane  of  the 
ecliptic.  They  cross,  going  south,  at  the  descend- 
ing node  ;  going  north,  at  the  ascending  node. 
When  we  see  them  apparently  crossing  the  eclip- 
tic on  the  celestial  sphere,  this  is  the  effect  of 
projection,  but  they  are  then  crossing  the  plane 
of  the  ecliptic,  102. 

Nomenclature  of  the  fixed  stars,  4. 

Nubecula  Major  and  Nubecula  Minor,  233. 

Obliquity  of  the  sun's  rays,  63-68. 

Occultation  of  stars  by  the  moon,  202  ;  of  Jupiter's 
mcons  by  Jupiter,  199. 

Old  style,  88,  89. 

Ophiuchus,  new  star  in,  229. 

Opposition.  A  planet  or  the  moon  is  in  opposi- 
tion with  the  sun  when  sun,  earth,  and  planet 
are  in  line  with  the  earth  in  the  middle,  95,  125, 
128. 

Orbits.  These  are  the  paths  of  bodies  revolving 
around  the  sun  or  some  planet,  39,  43,  46,  96, 
192,  193. 

Orion,  nebula  in,  232. 

Pallas,  minor  planet,  188. 

Parallactic  motions,  the  sun's  motion  north  and 
south,  28  ;  retrograde  motion  of  the  superior 
planets,  133-139  ;  secular  motion  of  fixed  stars 
toward  a  point  in  Hercules,  219. 

Parallax.  An  observer's  change  of  place  causes  a 
displacement  of  bodies  on  the  background  against 
which  he  sees  them.  This  is  called  parallax,  166  ; 
horizontal  parallax,  the  parallax  of  Mars,  Venus, 
and  the  moon,  166, 167  ;  annual  parallax  of  some 
fixed  stars,  224. 

Parallels.  Celestial  parallels  are  small  circles  par- 
allel to  the  ecliptic,  48  ;  parallel  lines  appear 
convergent,  44. 

Pavo,  the  Peacock,  southern  circumpolar  constel- 
lation, 235. 

Penumbra,  112. 

Perigee.  The  position  of  the  sun  or  moon  when 
at  their  least  distance  from  the  earth,  46. 

Perihelion.  This  is  the  point  nearest  the  sun's 
center,  on  the  orbit  of  a  heavenly  body  revolv- 
ing round  the  sun,  46. 

Perseids,  a  stream  of  revolving  meteors,  208. 

Perseus,  cluster  of  stars  in,  230. 

Perturbations  of  planetary  bodies,  160. 

Phases,  of  the  moon,  103  ;  of  Venus,  149  ;  of  Mars, 
149,  note ;  of  Mercury,  155  ;  of  Saturn's  rings, 
200. 

Phoenix,  a  southern  circumpolar  constellation, 
235. 


ASTRONOMY  BY  OBSERVATION. 


Photosphere,  the  sun's,  178-182. 

Planes,  of  ecliptic,  39, 40  ;  of  planetary  orbits,  139, 
144,  193- 

Planets,  defined  and  distinguished,  I,  118,  168  ; 
superior  and  inferior,  127,  140,  188  ;  major  and 
minor  planets,  187,  188  ;  primary  and  secondary, 
218  ;  motion  of  superior  planets  discussed,  120- 
140 ;  their  synodical  revolutions,  oppositions, 
and  conjunctions,  125 ;  their  proper  motions, 
126 ;  variation  in  apparent  size,  variation  in 
distance  from  opposition  to  conjunction,  127  ; 
sidereal  revolutions,  128  ;  period  of  revolutions, 
how  learned,  130;  appearances  discussed,  131, 
132  ;  their  retrograde  motions,  133-138  ;  sum- 
mary of  observations  of  superior  planets,  139 ; 
motions  of  inferior  planets  discussed,  140-156  ; 
their  apparent  daily  revolutions,  142  ;  proper 
motions,  143  ;  paths  among  the  stars,  and  the 
ecliptic,  144 ;  elongations,  145  ;  theory  of  mo- 
tions, 147  ;  their  conjunctions,  148  ;  their  phases, 
149  ;  variations  in  apparent  size,  146, 155  ;  tran- 
sits, 150  ,  movements  as  morning  stars,  151 ;  re- 
trograde motions,  152  ;  how  we  learn  their  sy- 
nodical and  sidereal  periods,  153,  154;  the  planets 
of  all  classes,  186-203  >  'heir  forms,  189  ;  diam- 
eters and  volumes,  190  ;  densities,  191  ;  orbits, 
192  ;  planes  of  orbits  and  distances  from  the 
sun,  193  ;  periods  of  revolutions,  194  ;  rotations, 
195  ;  telescopic  appearances  and  physical  con- 
ditions, 196-203. 

Pleiades,  the,  cluster  of  stars  in  Taurus,  230. 

Pointers,  the,  stars  in  Great  Dipper,  7. 

Poles,  the  celestial,  13  ;  elevated  and  depressed 
poles,  21  ;  revolution  of,  77. 

Polhymnia,  a  minor  planet,  its  orbit,  192. 

Pollux,  1st  m.  star,  5  ;  its  color,  223. 

Precession  of  equinoxes,  76—79. 

Procyon,  ist  m.  star,  5  ;  color  and  physical  consti- 
tution, 223. 

Prominences,  the  solar,  183-185  ;  similar  phe- 
nomena of  fixed  stars,  229. 

Quadrature.  A  planet  or  the  moon  is  in  quadra- 
ture with  the  sun  when  lines  drawn  from  the 
earth  to  the  sun  and  planet  make  a  right  angle. 
At  quadrature  the  heavenly  bodies  are  on  the 
meridian  at  sunrise  and  sunset.  The  time  of 
quadrature  is  half-way  between  opposition  and 
conjunction.  The  symbol  of  quadrature  is  D, 
202. 

Radiant  point,  208. 

Radius  vector,  159. 

Refraction,  164. 

Regulus,  1st  m.  star,  5  ;  motion  of,  222. 

Retrograde  motion  of  superior  planets,  133-139  ; 

of  inferior  planets,  152;  of  meteors,  208. 
Revolutions.    Apparent  diurnal  revolution  of  stars. 

8  ;  of  sun,  9  ;  ofmoon,  92  ;  of  planets,  125,  142  ; 

apparent  annual  revolution  of  stars,  24 ;  of  sun 

on  the  ecliptic,  47  ;  synodical  revolution  of  the 


moon,  94-98  ;  of  the  superior  planets,  125,  130, 
153,  194;  diurnal  rotation  of  earth  on  her  axis, 
10,  II  ;  annual  revolution  of  earth  round  the 
sun,  35,  36  ;  sidereal  revolution  of  moon,  97  ;  of 
the  planets,  128,  154,  194  ;  revolution  of  the 
moon's  nodes,  in  ;  of  the  equinoctial  points 
upon  the  ecliptic,  76  ;  of  perihelion  and  aphe- 
lion points  of  the  earth's  orbit,  75  ;  of  celestial 
poles,  76-79  ;  apparent  revolution  of  the  earth 
on  the  ecliptic  (as  seen  from  the  sun),  47  ;  revo- 
lution of  double  stars  around  a  center,  227. 

Rigel,  ist  m.  star,  5  ;  motion  of,  222. 

Right  ascension.  This  is  angular  distance  meas- 
ured east  on  the  ecliptic  from  the  first  point  of 
Aries,  that  is  from  the  intersection  of  the  eclip- 
tic and  equinoctial  in  Pisces,  18. 

Satellites,  a  name  applied  to  moons,  198-200. 

Saturn,  a  superior  and  major  planet,  how  to  find 
it,  120-124  I  comparative  distance  from  the  earth 
at  conjunction,  127  ;  at  opposition,  138  ;  Saturn's 
motions  as  a  superior  planet,  120-140  ;  his  form, 
189;  volume  and  diameter,  190;  density,  191  ; 
orbit,  192  ;  plane  of  orbit,  and  distance  from  the 
sun,  193  ;  periods  of  revolution,  194  ;  rotation, 
195  ;  rings,  moons,  telescopic  aspect,  and  physi- 
cal condition,  200 ;  attraction  for  comets,  212, 
213. 

Seasons,  the,  63-71. 

Sirius,  ist  m.  star,  5 ;  motion  of,  222  ;  physical 
constitution,  223. 

Solar  system,  the,  168. 

Solstices,  summer  and  winter,  49. 

Southern  Cross,  southern  circumpolar  constella- 
tion, 235. 

Spectra,  bright-lined,  173;  reversed  and  continu- 
ous, 174  ;  solar  spectrum,  177  ;  speccra  of  chro- 
mosphere and  solar  prominences,  182-184;  °f 
the  solar  corona,  185  ;  of  comets,  215  ;  of  fixed 
stars,  223  ;  of  nebula;,  232. 

Spectroscope,  description  of ,  1 75  ;  spectroscope  and 
motion,  176. 

Spica,  ist  m.  star,  5  ;  color  and  physical  constitu- 
tion, 223. 

Stars,  fixed,  definition,  i  ;  constellations  of,  2 ; 
magnitudes,  3  ;  nomenclature^;  apparent  diur- 
nal revolution,  8  ;  paths  of  diurnal  motion,  22, 
23  ;  enormous  distances  of,  41,  44 ;  secular  ap- 
parent motion  of  the  stars,  219 ;  secular  proper 
motion  of,  220 ;  star-drift,  221  ;  motion  of  prin- 
cipal stars  toward  or  from  us,  222  ;  physical  con- 
stitution of  the  stars  and  resemblance  to  the  sun, 
223  ;  annual  parallax  and  their  distances  from 
us,  224  ;  fixed  stars  in  telescope,  225  ;  numbers 
of  the  fixed  stars,  226 ;  double  and  multiple 
stars,  227  ;  variable  stars,  228  ;  new  stars,  229  ; 
star-clusters,  230  ;  nebulous  stars,  232. 

Sun,  the,  apparent  diurnal  motion  of,  9  ;  does  not 
make  annual  revolution,  25,  38  ;  solar  days,  26  ; 
apparent  motion  of  the  sun  north  and  south,  28  ; 
sun  stationary,  28,  50 ;  revolution  on  the  eclip- 
tic, 47  ;  measurement  of  the  sun's  annual  mo- 


tion on  the  ecliptic,  51  ;  variation  in  distance 
and  apparent  size,  43,  46  ;  unequal  motion  on 
the  ecliptic,  72  ;  eclipses  of,  115  ;  measurement 
of  distance  from  the  earth,  167,  193  ;  diameter, 
volume,  mass,  169  ;  light  of,  170;  heat  of,  171 ; 
telescopic  appearance,  178-186;  solar  spec- 
trum, 177  ;  chemical  elements  in  the  sun,  177  ; 
elements  in  chromosphere,  183,  184;  corona, 
185  ;  faculae  and  granules,  180 ;  sun-spots,  181  ; 
rotation  and  inclination  of  axis  to  the  plane  of 
the  ecliptic,  181 ;  secular  motion  of  the  sun, 
219  ;  comparison  of  the  sun  and  the  fixed  stars, 
223. 

Telescope,  the,  Appendix  B,  page  86. 

Temperature,  annual  change  of,  63-72  ;  tempera- 
ture and  length  of  days,  70. 

Terminator.  The  line  separating  the  daik  and 
illumined  portions  of  the  moon  is  called  the 
terminator,  202. 

Tides,  the,  163. 

Time,  or  the  calendar,  81-91. 

Titan,  moon  of  Saturn,  200. 

Toucan,  a  southern  circumpolar  constellation,  230, 

235- 

Transits  of  Venus,  150,  167  ;  of  the  moons  of  Ju- 
piter, 199. 

Tropics  of  Cancer  and  Capricorn,  62,  80. 

Tycho  Brahe,  159. 

Tycho,  a  crater  on  the  moon,  2O2. 

Umbra,  112. 

Uranus,  a  superior  and  major  planet,  discovery  of, 
187  ;  for  explanation  of  his  motions,  see  account 
of  superior  plants,  120-140  ;  volume  and  diame- 
ter, 190  ;  density,  191  ;  orbit,  192  ;  plane  of  or- 
bit and  distance  from  the  sun,  193  ;  period  of 
his  sidereal  and  synodical  revolutions,  194  ;  ro- 
tation, 195  ;  physical  condition  and  telescopic 
aspect,  moons,  201. 

Vega,  1st  m.  star,  5  ;  motion  of,  222  ;  color  and 
physical  constitution  of,  223. 

Venus,  an  inferior  and  major  planet,  140, 187  ;  how 
to  find  her,  141  ;  apparent  diurnal  revolution, 
142  ;  proper  motion,  143  ;  comparative  rate  of 
angular  motion,  143  ;  Venus  and  the  ecliptic, 
144  ;  her  elongations,  145  ;  variation  in  brill- 
iancy, 146 ;  explanation  of  her  motions,  147  ; 
her  conjunctions,  148 ;  her  phases,  149 ;  her 
transits,  150,  167  ;  Venus,  morning  star,  151  ;  her 
diameter  and  volume,  190  ;  density,  191  ;  orbit, 

192  ;  plane  of  orbit  and  distance  from   the  sun,  ' 

193  ;  time  of  revolution,    194  ;    rotation,    195  ; 
atmosphere  and  physical  condition,  197. 

Volumes  of  planets,  190. 

Year,  the,  tropical  and  sidereal,  81  ;  calendar  year, 
86-90  ;  leap-year,  86. 

Zenith.  This  is  the  point  of  the  celestial  sphere 
directly  over  the  observer's  head,  19. 


THE     END. 


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